Unforeseen small resistance infiltrating revolution (10)
I believe the answer is:
surprising
'unforeseen' is the definition.
(thesaurus)
'small resistance infiltrating revolution' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'resistance' becomes 'r' (abbreviation).
'infiltrating' is an insertion indicator.
'revolution' becomes 'uprising' (I've seen this before**).
'r' going within 'uprising' is 'urprising'.
's'+'urprising'='SURPRISING'
(Other definitions for surprising that I've seen before include "Causing astonishment" , "Unexpected" , "Mildly astonishing" .)