Spiny lobster, tuna and sole? Finally choosing flounders! (9)
I believe the answer is:
langouste
'spiny lobster' is the definition.
(synonyms)
'tuna and sole? finally choosing flounders' is the wordplay.
'and' means one lot of letters go next to another.
'finally' says to take the final letters.
'flounders' is an anagram indicator (I've seen 'floundering' mean this (letters flounder or move around)).
The last letter of 'choosing' is 'g'.
'sole'+'g'='soleg'
'tuna' put after 'soleg' is 'solegtuna'.
'solegtuna' with letters rearranged gives 'LANGOUSTE'.
(Other definitions for langouste that I've seen before include "Shellfish" , "One in the sea" , "Seafood is possibly" , "It may be eaten roasted" , "Swimmer" .)