Stain puts FBI agents in Hell (7)
I believe the answer is:
pigment
'stain' is the definition.
(thesaurus)
'fbi agents in hell' is the wordplay.
'fbi agents' becomes 'g-men' ('G-man' can be a synonym of 'fbi agent').
'in' means one lot of letters goes inside another.
'hell' becomes 'pit' ('the pit' can mean hell).
'gmen' inserted inside 'pit' is 'PIGMENT'.
'puts' is the link.
(Other definitions for pigment that I've seen before include "Substance giving colour" , "become imbued" , "Colourful substance" , "Colouring material" , "Colouring substance" .)