Pose with lettuce in buttonhole (6)
I believe the answer is:
accost
'buttonhole' is the definition.
(both can mean to detain in conversation)
'pose with lettuce' is the wordplay.
'pose' becomes 'act' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters goes inside another.
'lettuce' becomes 'cos' (cos lettuce is a kind of lettuce).
'act' enclosing 'cos' is 'ACCOST'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for accost that I've seen before include "Make bold approach to" , "Approach or confront boldly" , "Tackle" , "Come boldly up to" , "Approach boldly and speak to" .)