At sea, turn and row boat, catching one fish (7,5)
I believe the answer is:
rainbow trout
'fish' is the definition.
(I know that rainbow trout is a type of trout)
'at sea turn and row boat catching one' is the wordplay.
'at sea' indicates anagramming the letters.
'and' means one lot of letters go next to another.
'catching' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'turn'+'row'+'boat'='turnrowboat'
'turnrowboat' is an anagram of 'ranbowtrout'.
'ranbowtrout' enclosing 'i' is 'RAINBOW-TROUT'.
(Other definitions for rainbow trout that I've seen before include "Popular food fish" , "game swimmer" , "Finely coloured fish" .)