Myrtle blossoms three times a year (6)
I believe the answer is:
termly
'three times a year' is the definition.
(every school term)
'myrtle blossoms' is the wordplay.
'blossoms' indicates an anagram.
'myrtle' with letters rearranged gives 'TERMLY'.
(Other definitions for termly that I've seen before include "Per school session" , "school magazine?" , "every semester" , "In each division of the academic year" .)