Give money back to travelling salesman, certainly (5)
I believe the answer is:
repay
'give money back' is the definition.
'travelling salesman certainly' is the wordplay.
'travelling salesman' becomes 'rep' (I've seen this before).
'certainly' becomes 'ay' (variant spelling of 'aye').
'rep'+'ay'='REPAY'
'to' is the link.
(Other definitions for repay that I've seen before include "recompense" , "Do or give in return" , "Reimburse" , "Quite old" , "Return money borrowed" .)