Sticker permitted troublemaker to enter (6)
I believe the answer is:
limpet
'sticker' is the definition.
(thesaurus)
'permitted troublemaker to enter' is the wordplay.
'permitted' becomes 'let' (I've seen this before).
'troublemaker' becomes 'imp' (I've seen this before).
'to enter' indicates putting letters inside.
'let' placed around 'imp' is 'LIMPET'.
(Other definitions for limpet that I've seen before include "Mollusc adhering to rocks" , "he's not easily got rid of" , "Proverbially clinging shellfish" , "Adhesive mollusc" , "A shellfish that sticks fast" .)