Master a rebel leader for one short month (3)
I believe the answer is:
'month' is the definition.
(Mar is an example)
'master a rebel leader for one' is the wordplay.
'master' becomes 'm'.
'a rebel leader' becomes 'r' (I've seen this in other clues).
'for' says to put letters next to each other (I've seen this in other clues).
'one' becomes 'a' ().
'r' after 'a' is 'ar'.
'short' is the link.
(Other definitions for mar that I've seen before include "when spring growth starts?" , "Ruin" , "Blight" , "hurt" , "Oh dear!" .)