Amazement of criminal upset splinter group (12)
I believe the answer is:
stupefaction
'amazement' is the definition.
(stupefaction is a kind of amazement)
'criminal upset splinter group' is the wordplay.
'criminal' is an anagram indicator.
'splinter group' becomes 'faction' (as in a faction of an organisation).
'upset' is an anagram of 'stupe'.
'stupe'+'faction'='STUPEFACTION'
'of' is the link.
(Other definitions for stupefaction that I've seen before include "Total astonishment and shock" , "Extreme astonishment" , "great astonishment" , "Shocked astonishment" .)