Used a strop on leader of the flock (6)
I believe the answer is:
'leader of the flock' is the definition.
(I've seen this in another clue)
'used a strop on' is the wordplay.
'used' indicates an anagram.
'on' means one lot of letters go next to another.
'a' after 'strop' is 'stropa'.
'stropa' is an anagram of 'PASTOR'.
(Other definitions for pastor that I've seen before include "who may provide a service" , "Minister; shepherd" , "One with a flock (of people)" , "Minister of religion" , "Church minister" .)