Kiss Frenchman when in celebration (4)
I believe the answer is:
xmas
'celebration' is the definition.
(I have seen 'Short celebration' mean 'xmas' so perhaps 'celebration' could also mean 'xmas')
'kiss frenchman when' is the wordplay.
'kiss' becomes 'x' (X can represent a kiss).
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'x'+'m'+'as'='XMAS'
'in' acts as a link.
(Other definitions for xmas that I've seen before include "Christmas (informal)" , "Abbreviation for a winter festival" , "Yuletide" , "annual holiday" , "Brief version of the Nativity, beginning with Greek letter" .)