Unexpectedly grumpy about quiet revolution (12)
I believe the answer is:
surprisingly
'unexpectedly' is the definition.
(I've seen this before)
'grumpy about quiet revolution' is the wordplay.
'grumpy' becomes 'surly'.
'about' means one lot of letters goes inside another.
'quiet' becomes 'p' (music symbol for 'piano').
'revolution' becomes 'rising'.
'p'+'rising'='prising'
'surly' placed around 'prising' is 'SURPRISINGLY'.
(Other definitions for surprisingly that I've seen before include "Amazingly" , "Unexpectedly" .)