About to transgress, by gum! (5)
I believe the answer is:
resin
'gum' is the definition.
(thesaurus)
'about to transgress by' is the wordplay.
'about' becomes 're' (regarding**).
'to transgress' becomes 'sin' (sinning is a kind of transgressing).
'by' means one lot of letters go next to another.
're'+'sin'='RESIN'
(Other definitions for resin that I've seen before include "Substance obtained from sap of certain plants - transgress again" , "Material exuded from tree" , "Organic polymer used in varnishes, adhesives etc." , "plastic material" , "stuff that's sticky" .)