A reduced flavouring in jelly (5)
I believe the answer is:
aspic
'jelly' is the definition.
(aspic is a kind of jelly)
'a reduced flavouring' is the wordplay.
'reduced' means to remove the last letter.
'flavouring' becomes 'spice' (spice is a kind of flavouring).
'spice' with its last letter removed is 'spic'.
'a'+'spic'='ASPIC'
'in' acts as a link.
(Other definitions for aspic that I've seen before include "Savoury jelly for meat or vegetable" , "Edible jelly" , "Savoury jelly used to coat eggs etc" , "Jelly made with fish or meat stock" , "Gelatin product" .)