Stigmatise ring-leader in gang (5)
I believe the answer is:
brand
'stigmatise' is the definition.
('brand' can be a synonym of 'stigmatize')
'ring-leader in gang' is the wordplay.
'leader' suggests taking the first letters.
'in' indicates putting letters inside.
'gang' becomes 'band' (I've seen this before).
The initial letter of 'ring' is 'r'.
'r' going within 'band' is 'BRAND'.
(Other definitions for brand that I've seen before include "Mark (by burning)" , "Mark with a hot iron" , "Type of product - identifying mark on livestock" , "Trademark - burn with hot iron" , "Stamp" .)