Buttonhole (6)
I believe the answer is:
accost
'buttonhole' is the definition.
(both can mean to detain in conversation)
This is the entire clue.
(Other definitions for accost that I've seen before include "Approach boldly or aggressively" , "Approach boldly and speak to" , "Approach and address (someone) aggressively" , "Buttonhole" , "Tackle" .)