Mark, during Christmas, served up fruit (5)
I believe the answer is:
lemon
'fruit' is the definition.
(I know that lemon is a type of citrus fruit)
'mark during christmas served up' is the wordplay.
'mark' becomes 'm'.
'during' is an insertion indicator.
'christmas' becomes 'Noel' (Noel can mean Christmas).
'served up' is a reversal indicator (in a down clue, going upwards).
'm' inserted within 'noel' is 'nomel'.
'nomel' back-to-front is 'LEMON'.
(Other definitions for lemon that I've seen before include "Fruit - as an answer it's unsatisfactory" , "stinker" , "something unattractive" , "Defective object" , "Citrus fruit (is it the answer?)" .)