It's smart catching fish that's drifting (8)
I believe the answer is:
straying
'drifting' is the definition.
('stray' can be a synonym of 'drift')
'it's smart catching fish' is the wordplay.
'it's smart' becomes 'sting' (stinging is a kind of smarting**).
'catching' means one lot of letters goes inside another.
'fish' becomes 'ray' (ray is a kind of fish).
'sting' enclosing 'ray' is 'STRAYING'.
'that's' acts as a link.
(Other definitions for straying that I've seen before include "Wandering from a direct course" , "Errant" , "Getting sidetracked" , "aberration" , "Drifters may be" .)