Kiss 1000 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 1000 when' is the wordplay.
'kiss' becomes 'x' (X can represent a kiss).
'1000' becomes 'M' (roman numeral).
'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)" , "present time" , "Yuletide" , "Time to give" , "December holiday..." .)