Taste last bit of tuna in dish (6)
I believe the answer is:
palate
'taste' is the definition.
(palate can mean an ability to distinguish tastes)
'last bit of tuna in dish' is the wordplay.
'last bit of tuna' becomes 'al' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'dish' becomes 'pate' (I've seen this in other clues).
'al' inserted inside 'pate' is 'PALATE'.
(Other definitions for palate that I've seen before include "part of trap" , "having ability to appreciate good food" , "relish" , "Liking" , "Sense of taste at roof of mouth" .)