The most common and most likely answer to this crossword clue is the 5 letter word BASIS
I've seen this crossword clue in The Times
B
A
S
I
S
More clues leading to the result BASIS
- Current breaking up fish grounds
- Fish one consumed as main ingredient
- Singer touring one’s grounds
- Fundamental principle to inject fish with iodine
- The fundamental ingredient of book without any changes
- Underlying principle, one accepted by male singer
- Singer tours one ground
- Foundation degree son is chasing
- Reason a follower has not changed
- The fundamental ingredient of cobbler essentially unaltered
Buddy explains!

The straight part
We know that "Reason" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reason = basis.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 BASIS mean?
BASIS noun- A physical base or foundation.
- A starting point, base or foundation for an argument or hypothesis.
- An underlying condition or circumstance.
- A regular frequency.
- In a vector space, a linearly independent set of vectors spanning the whole vector space.
- Amount paid for an investment, including commissions and other expenses.
- A collection of subsets ("basis elements") of a set, such that this collection covers the set, and for any two basis elements which both contain an element of the set, there is a third basis element contained in the intersection of the first two, which also contains that element.