Unexpected, small rebellion involving Republican (10)
I believe the answer is:
surprising
'unexpected' is the definition.
(associated in meaning)
'small rebellion involving republican' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'rebellion' becomes 'uprising' (synonyms).
'involving' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'uprising' enclosing 'r' is 'urprising'.
's'+'urprising'='SURPRISING'
(Other definitions for surprising that I've seen before include "Causing astonishment" , "Unexpected" , "Mildly astonishing" .)