Let out while upset (4)
I believe the answer is:
emit
'let out' is the definition.
(I know that let out can be written as emit)
'while upset' is the wordplay.
'while' becomes 'time' (I've seen this before).
'upset' says the letters should be written backwards.
'time' back-to-front is 'EMIT'.
(Other definitions for emit that I've seen before include "'Give off, odour say (4)'" , "Sent" , "Send forth" , "Expel" , "Give off, heat say" .)