Send payment for retro stopwatch (5)
I believe the answer is:
remit
'send payment' is the definition.
(I've seen this before)
'retro stopwatch' is the wordplay.
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'stopwatch' becomes 'timer' (I've seen this in another clue).
'timer' back-to-front is 'REMIT'.
'for' acts as a link.
(Other definitions for remit that I've seen before include "Pardon or transmit money" , "Timer (anag.)" , "Abate; send (money)" , "Job specification" , "Dispatch" .)