Fruit marmalade primarily filling Christmas turnover (5)
I believe the answer is:
lemon
'fruit' is the definition.
(lemon is a kind of fruit)
'marmalade primarily filling christmas turnover' is the wordplay.
'primarily' indicates taking the first letters.
'filling' indicates putting letters inside.
'christmas' becomes 'noel' (synonyms).
'turnover' shows that the letters should be reversed in order (the letters turn over).
The first letter of 'marmalade' is 'm'.
'm' put within 'noel' is 'nomel'.
'nomel' back-to-front is 'LEMON'.
(Other definitions for lemon that I've seen before include "Small evergreen fruit tree" , "Sharp-tasting fruit" , "something defective" , "Strong yellow colour" , "Washout" .)