Working around the setter is a sign (4)
I believe the answer is:
omen
'is a sign' is the definition.
(I've seen this before)
'working around the setter' is the wordplay.
'working' becomes 'on'.
'around' indicates putting letters inside.
'the setter' becomes 'me' (the setter of the clue).
'on' going around 'me' is 'OMEN'.
(Other definitions for omen that I've seen before include "Portent of things to come" , "Portent, prophetic sign" , "Sign of something about to happen" , "Augur" , "Augury, portent" .)