A flavouring, mostly jelly (5)
I believe the answer is:
aspic
'jelly' is the definition.
(aspic is a kind of jelly)
'a flavouring mostly' is the wordplay.
'flavouring' becomes 'spice' (spice is a kind of flavouring).
'mostly' means to remove the last letter.
'spice' with its last letter taken off is 'spic'.
'a'+'spic'='ASPIC'
(Other definitions for aspic that I've seen before include "Edible jelly" , "Glaze" , "Savoury jelly used to coat eggs etc" , "Savoury meat or fish jelly" , "Clear savoury jelly" .)