Send up instructions to make part of explosive (5)
I believe the answer is:
timer
'instructions to make part of explosive' is the definition.
I can't judge whether this defines the answer.
'send up' is the wordplay.
'send' becomes 'remit' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'remit' back-to-front is 'TIMER'.
Can you help me to learn more?
(Other definitions for timer that I've seen before include "One may be alarmed" , "Maybe watch" , "one of 14 [CUCKOO CLOCKS]" , "chronometer" , "Clock or watch" .)