The most common and most likely answer to this crossword clue is the 5 letter word PRIME
I've seen this crossword clue in The Guardian
P
R
I
M
E
More clues leading to the result PRIME
- Eg two or three runs I’m to get in physical exercise
- First class permit almost misplaced
- Best school book, almost
- Prepare to be original
- Best quality piano introducing old poem
- Prepare to paint very formal European
- Nine is the most popular time for broadcasters
- Like 3, 13 and 17, in first of poems by Frost
- Prepare to fire two or three, for instance
- Press release initially upset EMI chief
Buddy explains!

The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PRIME mean?
PRIME noun- The first hour of daylight; the first canonical hour.
- The religious service appointed to this hour.
- The early morning generally.
- The earliest stage of something.
- The most active, thriving, or successful stage or period.
- The chief or best individual or part.
- The first note or tone of a musical scale.
- The first defensive position, with the sword hand held at head height, and the tip of the sword at head height.
- A prime element of a mathematical structure, particularly a prime number.
- A four-card hand containing one card of each suit in the game of primero; the opposite of a flush in poker.
- Six consecutive blocks, which prevent the opponent's pieces from passing.
- The symbol ′ used to indicate feet, minutes, derivation and other measures and mathematical operations.
- Any number expressing the combining weight or equivalent of any particular element; so called because these numbers were respectively reduced to their lowest relative terms on the fixed standard of hydrogen as 1.
- An inch, as composed of twelve seconds in the duodecimal system.
- The priming in a flintlock.
- Contraction of prime lens, a film lens
- First in importance, degree, or rank.
- First in time, order, or sequence.
- First in excellence, quality, or value.
- (lay) Having exactly two integral factors: itself and unity (1 in the case of integers).
- Such that if it divides a product, it divides one of the multiplicands.
- Having its complement closed under multiplication: said only of ideals.
- Marked or distinguished by the prime symbol.
- Early; blooming; being in the first stage.
- Lecherous, lewd, lustful.
- To prepare a mechanism for its main work.
- To apply a coat of primer paint to.
- To be renewed.
- To serve as priming for the charge of a gun.
- (of a steam boiler) To work so that foaming occurs from too violent ebullition, which causes water to become mixed with, and be carried along with, the steam that is formed.
- To apply priming to (a musket or cannon); to apply a primer to (a metallic cartridge).
- To prepare; to make ready; to instruct beforehand; to coach.
- To trim or prune.
- To mark with a prime mark.
- An intermediate sprint within a race, usually offering a prize and/or points.