Danger briefly upset teacher (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'danger briefly upset' is the wordplay.
'danger' becomes 'risk' (synonyms).
'briefly' means to remove the last letter.
'upset' is a reversal indicator.
'risk' with its last letter taken off is 'ris'.
'ris' backwards is 'SIR'.
(Other definitions for sir that I've seen before include "Title of 1 across" , "as Winston Churchill was" , "Gentleman's form of address" , "Knight's prefix" , "Term of address for a gentleman" .)