Deep, deep fish
I believe the answer is:
sea bass
'fish' is the definition.
(sea bass is a kind of fish)
'deep deep' is the wordplay.
'deep' becomes 'sea' (I've seen this before).
'deep' becomes 'bass' ('bass' can be a synonym of 'deep').
'sea'+'bass'='SEA-BASS'
(Other definitions for sea bass that I've seen before include "Edible marine fish" , "one showing off plenty of scales" , "Marine food fish" , "Saltwater fish" .)