Hard fruit served up with a fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'hard fruit served up with a' is the wordplay.
'hard fruit' becomes 'nut' (I've seen this in another clue).
'served up' says the letters should be written in reverse (in a down clue, going upwards).
'with' is a charade indicator (letters next to each other).
'nut' reversed gives 'tun'.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Tinned food fish" , "Common food fish" , "Bluefin, e.g." , "Widely-eaten fish" , "Skipjack --, blue-fin --" .)