Unexpected second rebellion involving leader of rebels (10)
I believe the answer is:
surprising
'unexpected' is the definition.
('surprising' is associated in meaning with 'unexpected')
'second rebellion involving leader of rebels' is the wordplay.
'second' becomes 's' (common abbreviation).
'rebellion' becomes 'uprising' ('uprising' can be a synonym of 'rebellion'**).
'involving' is an insertion indicator.
'leader of' says to take the initial letters.
The initial letter of 'rebels' is 'r'.
'uprising' placed around 'r' is 'urprising'.
's'+'urprising'='SURPRISING'
(Other definitions for surprising that I've seen before include "Causing astonishment" , "Unexpected" , "Mildly astonishing" .)