Rebel is compelling, knight admitted (9)
I believe the answer is:
insurgent
'rebel' is the definition.
(synonyms)
'is compelling knight admitted' is the wordplay.
'compelling' becomes 'urgent' (I've seen this before).
'knight' becomes 'n' (chess).
'admitted' means one lot of letters goes inside another.
'is'+'urgent'='isurgent'
'isurgent' going around 'n' is 'INSURGENT'.
(Other definitions for insurgent that I've seen before include "Unresting like a rebel" , "Rebel, revolutionary" , "rising" , "Revolting" , "freedom fighter" .)