Fish kept by harbour (7)
I believe the answer is:
haddock
'fish' is the definition.
(haddock is a kind of fish)
'kept by harbour' is the wordplay.
'kept' becomes 'had' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'harbour' becomes 'dock' (I've seen this before).
'had'+'dock'='HADDOCK'
(Other definitions for haddock that I've seen before include "Popular food fish" , "Fish like cod but smaller" , "Cod-like fish" , "Fish similar to but smaller than cod" , "Swimmer" .)