Off-colour setter taking bait occasionally — unlucky (3-5)
I believe the answer is:
ill-fated
'unlucky' is the definition.
(I know that unlucky can be written as ill-fated)
'off-colour setter taking bait occasionally' is the wordplay.
'off colour' becomes 'ill' (off-colour can mean unwell).
'setter' becomes 'fed' (Fed is a Guardian crossword setter).
'taking' indicates putting letters inside.
'occasionally' indicates alternate letters.
The alternating letters of 'bait' are 'at'.
'fed' enclosing 'at' is 'fated'.
'ill'+'fated'='ILL-FATED'
(Other definitions for ill-fated that I've seen before include "Doomed(3.5)" , "Destined to fail" , "Yelp like pig" , "Unlucky, bad times ahead" , "Unlucky, destined to lose" .)