Session spanning winter months (4)
I believe the answer is:
term
'session' is the definition.
(thesaurus)
'spanning winter months' is the wordplay.
'spanning' indicates the answer is hidden within the clue (the solution spans multiple words in the clue).
'TERM' can be found hidden inside 'winter months'.
(Other definitions for term that I've seen before include "Limit - school time" , "Word; trimester" , "Time spent at school" , "Fixed period or special word" , "sentence?" .)