Send back clock? (5)
I believe the answer is:
timer
'clock?' is the definition.
(something which times)
'send back' is the wordplay.
'send' becomes 'remit' (I've seen this before).
'back' is a reversal indicator.
'remit' written backwards gives 'TIMER'.
(Other definitions for timer that I've seen before include "Preset activator" , "One may have the minutes" , "hourglass" , "Remit for, say, measuring device - or the measurer (5)" , "chronometer" .)