Send out a signal when the time is up (4)
I believe the answer is:
emit
'send out' is the definition.
(I've seen this in another clue)
'time is up' is the wordplay.
'is up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'time' back-to-front is 'EMIT'.
'a signal when the' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for emit that I've seen before include "Throw out" , "'Give off, odour say (4)'" , "Give out (eg light)" , "Issue" , "Sent" .)