Filling in canapes, tomato sauce! (5)
I believe the answer is:
pesto
'sauce' is the definition.
(pesto is a type of sauce)
'filling in canapes tomato' is the wordplay.
'filling in' says the answer is hidden in the clue.
'PESTO' can be found hidden inside 'canapes tomato'.
(Other definitions for pesto that I've seen before include "Green sauce" , "Pasta sauce mainly of basil and cheese" , "Ligurian food" , "Saucy work" , "A sauce served with pasta" .)