Guy's initial anxiety on about criminal (8)
I believe the answer is:
gangster
'criminal' is the definition.
(I know that gangster is a type of criminal)
'guy's initial anxiety on about' is the wordplay.
'initial' indicates taking the first letters.
'anxiety' becomes 'angst' (angst is a type of anxiety).
'on' becomes 're' ('on' can be mean 'with reference to').
'about' says the letters should be written backwards.
The initial letter of 'guys' is 'g'.
're' backwards is 'er'.
'g'+'angst'+'er'='GANGSTER'
(Other definitions for gangster that I've seen before include "Soprano possibly" , "Wise guy" , "Organised criminal" , "Hoodlum" , "Member of a band of criminals" .)