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 in another clue).
'back' shows that the letters should be reversed in order.
'remit' in reverse letter order is '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" .)