Kiss on Mother's Day — in December? (4)
I believe the answer is:
xmas
'day in december?' is the definition.
'kiss on mother's' is the wordplay.
'kiss' becomes 'x'.
'on' means one lot of letters go next to another.
'mother's' becomes 'mas' (ma is a kind of mother).
'x'+'mas'='XMAS'
(Other definitions for xmas that I've seen before include "Brief version of the Nativity, beginning with Greek letter" , "Time to give" , "Festive season (abbr)" , "Yule, Noel" , "Winter festival (informal)" .)