A seasoning mostly found in jelly (5)
I believe the answer is:
aspic
'jelly' is the definition.
(aspic is a kind of jelly)
'a seasoning mostly' is the wordplay.
'seasoning' becomes 'spice' (spice is a kind of seasoning).
'mostly' means to remove the last letter.
'spice' with its final letter taken off is 'spic'.
'a'+'spic'='ASPIC'
'found in' acts as a link.
(Other definitions for aspic that I've seen before include "Savoury meat or fish jelly" , "stock production" , "Setter" , "Preservative jelly" , "Savoury jelly for coating meats and eggs" .)