Rebel leader mostly harsh about our lot (9)
I believe the answer is:
spartacus
'rebel leader' is the definition.
(Spartacus led a slave revolt)
'mostly harsh about our lot' is the wordplay.
'mostly' means to remove the last letter.
'harsh' becomes 'spartan' (spartan can mean harsh or austere).
'about' becomes 'c' (abbreviation for circa).
'our lot' becomes 'us'.
'spartan' with its final letter taken off is 'sparta'.
'sparta'+'c'+'us'='SPARTACUS'
(Other definitions for spartacus that I've seen before include "revolutionary leader" , "Rebellious Roman slave. d. c.71BC" , "Roman revolutionary" , "Leader of a gladiators' revolt in 73BC" , "Thracian who led the 73 BC slave revolt against Rome" .)