Leader’s first performance is curtailed (7)
I believe the answer is:
premier
'leader's' is the definition.
(premier is a kind of leader)
'first performance is curtailed' is the wordplay.
'first performance' becomes 'premiere' (I've seen this in another clue).
'is curtailed' means to remove the last letter.
'premiere' with its final letter removed is 'PREMIER'.
(Other definitions for premier that I've seen before include "Aberdeen or Salisbury?" , "man who's in charge" , "First in importance" , "Chief minister" , "Leading" .)