Marks volunteers getting involved in crimes (6)
I believe the answer is:
stains
'marks' is the definition.
('stain' can be a synonym of 'mark')
'volunteers getting involved in crimes' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'getting involved in' indicates putting letters inside.
'crimes' becomes 'sins'.
'ta' put inside 'sins' is 'STAINS'.
(Other definitions for stains that I've seen before include "Discolours or dyes wood" , "Discolorations on satins" , "Black marks for saints" , "Discolorations or dirt marks" , "Dirty marks" .)