Like one sort of fish apart (5)
I believe the answer is:
aside
'apart' is the definition.
(I know that apart can be written as aside)
'like one sort of fish' is the wordplay.
'like' becomes 'as'.
'one sort of fish' becomes 'ide' (ide is a type of fish).
'as'+'ide'='ASIDE'
(Other definitions for aside that I've seen before include "Words spoken apart in an undertone" , "Actor's line directed solely to audience" , "(Pushed) out of the way" , "Separately" , "(Turn) off main path" .)