They may be caught at sea and obtained on wharf (7)
I believe the answer is:
haddock
'they may be caught at sea' is the definition.
'obtained on wharf' is the wordplay.
'obtained' becomes 'had'.
'on' says to put letters next to each other.
'wharf' becomes 'dock' ('dock' can be a synonym of 'wharf').
'had'+'dock'='HADDOCK'
'and' acts as a link.
(Other definitions for haddock that I've seen before include "ish" , "Popular food fish" , "Fish, often smoked" , "White fish" , "Fish similar to but smaller than cod" .)