A season mostly for jelly (5)
I believe the answer is:
aspic
'jelly' is the definition.
(aspic is a kind of jelly)
'a season mostly' is the wordplay.
'season' becomes 'spice' (spicing is a kind of seasoning).
'mostly' means to remove the last letter (most but not all of the word).
'spice' with its last letter taken off is 'spic'.
'a'+'spic'='ASPIC'
'for' acts as a link.
(Other definitions for aspic that I've seen before include "Clear meat or fish jelly" , "Savoury jelly used to coat eggs etc" , "stock production" , "Savoury jelly for coating meats and eggs" , "Savoury jelly for coating meat, eggs etc" .)