The most common and most likely answer to this crossword clue is the 9 letter word TRIMESTER
I've seen this crossword clue in The Guardian
T
R
I
M
E
S
T
E
R
More clues leading to the result TRIMESTER
- Examiner buttons lip for a term
- This term oddly tires me out
- Merit rest — could be three months
- Someone trying to bag skirt for school term
- Part of year depicted in end of sonnet by old poet
- Margin accepted by one doing trial period
- Period when expecting conclusion from beat poet
- Examiner has sealed lip in academic period
- Three months guarding border’s a tricky problem
- A period when expecting to prune trees anew
Buddy explains!

The straight part
We know that "Three months" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case three months = trimester.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 TRIMESTER mean?
TRIMESTER noun- A period of three months or about three months; quarter.
- One of the terms of an academic year in those learning institutions that divide their teaching in three roughly equal terms, each about three months long. Compare semester.