Period in winter months (4)
I believe the answer is:
term
'period' is the definition.
(term is a kind of period)
'in winter months' is the wordplay.
'in' says the answer is hidden in the clue.
'TERM' is hidden within 'winter months'.
(Other definitions for term that I've seen before include "Word or expression and period of time" , "Academic period" , "Limit - school time" , "School time; word" , "Expression, or period in which law court holds sessions" .)