Turn down an opportunity, perhaps, to join again (6)
I believe the answer is:
refuse
This clue is a double definition.
'turn down an opportunity perhaps' is the first definition.
'join again' is the second definition.
(as in to re-fuse)
'to' acts as a link.
(Other definitions for refuse that I've seen before include "Decline to accept - rubbish" , "Pass up" , "Deny; rubbish" , "litter" , "Decline to jump (a fence)" .)