Corsairs upset a priest (7)
I believe the answer is:
pirates
'corsairs' is the definition.
(I've seen this in another clue)
'upset a priest' is the wordplay.
'upset' indicates an anagram.
'a'+'priest'='apriest'
'apriest' with letters rearranged gives 'PIRATES'.
(Other definitions for pirates that I've seen before include "Robbers at sea" , "More than one brigand" , "Sea trip (anag.)" , "Chatters, talks foolishly" , "Robbers on the high seas" .)