Gunman maybe spinning barrel before start of roulette (8)
I believe the answer is:
murderer
'gunman' is the definition.
(thesaurus)
'spinning barrel before start of roulette' is the wordplay.
'spinning' says the letters should be written backwards.
'barrel' becomes 'drum' (synonyms).
'before' becomes 'ere' (poetic term for 'before').
'start of' indicates taking the first letters.
The initial letter of 'roulette' is 'r'.
'drum' back-to-front is 'murd'.
'murd'+'ere'+'r'='MURDERER'
'maybe' acts as a link.
(Other definitions for murderer that I've seen before include "Criminal killer" , "One commits homicide" , "Serious criminal enters demurrer" , "Deliberate unlawful killer" , "hit man" .)