Unfortunately containing fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a kind of fish)
'unfortunately containing' is the wordplay.
'containing' says the answer is hidden in the clue (I've seen 'contains' mean this).
'TUNA' is hidden in the letters of 'unfortunately'.
(Other definitions for tuna that I've seen before include "(Meat of) large fish" , "Tunny" , "Tinned food fish" , "Fish; edible cactus" , "Skipjack --, blue-fin --" .)