The most common and most likely answer to this crossword clue is the 6 letter word HEYDAY
I've seen this crossword clue in The Times
H
E
Y
D
A
Y
More clues leading to the result HEYDAY
- Prime fodder keeping colour up
- Prime suspect, yeh, Doris
- Prime gay rhymes
- Perhaps heady years to begin with?
- Prime tips for hastily covering a stain up
- They’d a yearning to preserve golden age
- Golden era when colour returns in dried grass
- Period of vigour brings heady changes before the end of May
- They head off with 24 hours to reach the peak
- A man wise in conversation around daughter on American prime time?
Buddy explains!

The straight part
We know that "Prime" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prime = heyday.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 HEYDAY mean?
HEYDAY noun- A period of success, popularity, or power; prime.
- An exultation of the spirits; gaiety; frolic.
- A lively greeting.
- An expression of frolic and exultation, and sometimes of wonder.