Bug trapper about to trespass (5)
I believe the answer is:
resin
'bug trapper' is the definition.
(I've seen this in another clue)
'about to trespass' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to trespass' becomes 'sin' (I've seen this in another clue).
're'+'sin'='RESIN'
(Other definitions for resin that I've seen before include "stuff that's sticky" , "Fiddler's need" , "eg Rosin" , "Very important part of plastics industry" , "Substance from tree-sap" .)