An acquaintance may swindle before diplomacy (7)
I believe the answer is:
contact
'an acquaintance' is the definition.
(I have seen 'Useful acquaintance' mean 'contact' so perhaps 'acquaintance' could also mean 'contact')
'swindle before diplomacy' is the wordplay.
'swindle' becomes 'con' (con is a kind of swindle).
'before' says to put letters next to each other.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'con'+'tact'='CONTACT'
'may' acts as a link.
(Other definitions for contact that I've seen before include "Close interaction" , "Communication" , "Talk to" , "Meeting" , "securing current passage" .)