A very filling bitter taste (6)
I believe the answer is:
savour
'taste' is the definition.
(savouring is a kind of tasting)
'a very filling bitter' is the wordplay.
'a very' becomes 'av' (I've seen this in other clues).
'filling' means one lot of letters goes inside another.
'bitter' becomes 'sour' (I've seen this before).
'av' inserted within 'sour' is 'SAVOUR'.
(Other definitions for savour that I've seen before include "Smack" , "Enjoy the flavour" , "Relish, enjoy" , "Characteristic taste" , "Taste, relish" .)