Cunning one in publicity rubbish (6)
I believe the answer is:
adroit
'cunning' is the definition.
(thesaurus)
'one in publicity rubbish' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'publicity' becomes 'ad' (advert).
'rubbish' becomes 'rot' (I've seen this before).
'ad'+'rot'='adrot'
'i' going into 'adrot' is 'ADROIT'.
(Other definitions for adroit that I've seen before include "Nimble and skilful" , "Clever, skilful" , "Skilful or ingenious" , "Dexterous, artful" , "Deft and skilful" .)