Still mostly very thirsty, having eaten a seafood dish (8)
I believe the answer is:
calamari
'seafood dish' is the definition.
(squid dish)
'still mostly very thirsty having eaten a' is the wordplay.
'still' becomes 'calm' (similar in meaning).
'mostly' means to remove the last letter.
'very thirsty' becomes 'arid' (arid means dry or thirsty).
'having eaten' is an insertion indicator.
'arid' with its last letter taken away is 'ari'.
'calm'+'ari'='calmari'
'calmari' placed around 'a' is 'CALAMARI'.
(Other definitions for calamari that I've seen before include "seafood" , "Mediterranean cooked squid" , "Battered squid rings" , "Squid served as food" , "Dish of squid" .)