The most common and most likely answer to this crossword clue is the 6 letter word MOTIVE
I've seen this crossword clue in The Guardian
M
O
T
I
V
E
More clues leading to the result MOTIVE
- Reason to propose holding it back
- Reason second up-and-coming musical is cut short
- Persuade to keep it up, offering inducement
- Reason to relocate around half- time
- Reason I have to be seen by doctor – temperature
- Reject the origins of theory of mind? I have a reason
- Inducement of sex rejected in advance
- 27 shift across this, oddly
- Rationale of one very involved in scrap
- Thinking it over in advance?
Buddy explains!

The straight part
We know that "a reason" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a reason = motive.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 MOTIVE mean?
MOTIVE noun- An idea or communication that makes one want to act, especially from spiritual sources; a divine prompting.
- An incentive to act in a particular way; a reason or emotion that makes one want to do something; anything that prompts a choice of action.
- A limb or other bodily organ that can move.
- Something which causes someone to want to commit a crime; a reason for criminal behaviour.
- A motif.
- A motif; a theme or subject, especially one that is central to the work or often repeated.
- To prompt or incite by a motive or motives; to move.
- Causing motion; having power to move, or tending to move
- Relating to motion and/or to its cause