Nail leader of rowdies in criminal environment (4)
I believe the answer is:
brad
'nail' is the definition.
(brad is a kind of nail)
'leader of rowdies in criminal environment' is the wordplay.
'leader of' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'criminal environment' becomes 'bad' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'rowdies' is 'r'.
'r' inserted inside 'bad' is 'BRAD'.
(Other definitions for brad that I've seen before include "One to get hammered" , "Small-headed nail" , "Small tapered nail" , "One may secure" , "Fixer" .)