Going off fish netted in swindle (8)
I believe the answer is:
straying
'going off' is the definition.
'fish netted in swindle' is the wordplay.
'fish' becomes 'ray' (ray is a kind of fish).
'netted in' is an insertion indicator.
'swindle' becomes 'sting' (sting is a kind of swindle).
'ray' inserted within 'sting' is 'STRAYING'.
(Other definitions for straying that I've seen before include "aberration" , "Wandering from a direct course" , "Getting sidetracked" , "Being bad" , "not on target" .)