Rep is suave, terribly convincing (10)
I believe the answer is:
persuasive
'convincing' is the definition.
(associated in meaning)
'rep is suave terribly' is the wordplay.
'terribly' is an anagram indicator.
'rep'+'is'+'suave'='repissuave'
'repissuave' anagrammed gives 'PERSUASIVE'.
(Other definitions for persuasive that I've seen before include "Convincing" , "Providing sound reasoning" .)