Watch, say, brief getting upset (5)
I believe the answer is:
timer
'watch' is the definition.
(a watch might be used to time something)
'brief getting upset' is the wordplay.
'brief' becomes 'remit' (both can mean someone's responsibilities).
'getting upset' is a reversal indicator.
'remit' written backwards gives 'TIMER'.
'say' is the link.
(Other definitions for timer that I've seen before include "Stopwatch, say" , "one of 14 [CUCKOO CLOCKS]" , "chronometer" , "Trigger mechanism" , "Clock, say" .)