Dull American Democrat arrested by G-man (5)
I believe the answer is:
faded
'dull' is the definition.
(I've seen this in another clue)
'american democrat arrested by g-man' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'democrat' becomes 'd'.
'arrested by' is an insertion indicator (inserted letters are held or arrested).
'g-man' becomes 'fed' (I've seen this in another clue).
'a'+'d'='ad'
'ad' put inside 'fed' is 'FADED'.
(Other definitions for faded that I've seen before include "Lost freshness or brilliance" , "Bleached or deteriorated" , "Lost youth" , "Gradually disappeared" , "Washed-out, dimmed" .)