Send back this clock (5)
I believe the answer is:
timer
'this clock' is the definition.
(thesaurus)
'send back' is the wordplay.
'send' becomes 'remit' (I've seen this before).
'back' is a reversal indicator.
'remit' in reverse letter order is 'TIMER'.
(Other definitions for timer that I've seen before include "one of 14 [CUCKOO CLOCKS]" , "Pre-set switch" , "One may have the minutes" , "Clocklike device" , "It measures intervals" .)