Two short rebels backing North African Muslim (6)
'north african muslim' is the definition.
(I've seen this before)
'two short rebels backing' is the wordplay.
'two' says to double the letters.
'short' means to remove the last letter.
'rebels' becomes 'Rebs' ('Reb' can be a synonym of 'rebel').
'backing' says the letters should be written in reverse.
'rebs' with its last letter removed is 'reb'.
'reb' in reverse letter order is 'ber'.
'ber' duplicated is 'BERBER'.
(Other definitions for berber that I've seen before include "African type", "from North Africa", "North African Muslim", "North African native", "African people".)