Send up clock? (5)
I believe the answer is:
timer
'clock?' is the definition.
(something which times)
'send up' is the wordplay.
'send' becomes 'remit' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'remit' reversed gives 'TIMER'.
(Other definitions for timer that I've seen before include "Pre-set switch" , "Remit for, say, measuring device - or the measurer (5)" , "Clock, say" , "Trigger mechanism" , "Recorder of duration" .)