Assistant has a fish (4)
I believe the answer is:
aide
'assistant' is the definition.
(aide is a kind of assistant)
'a fish' is the wordplay.
'fish' becomes 'ide' (type of fish).
'a'+'ide'='AIDE'
'has' is the link.
(Other definitions for aide that I've seen before include "Political assistant" , "Officer's assistant" , "Somebody helping" , "Idea (anag)" , "Personal assistant in the military" .)