Saw agent dressed in brown (6)
I believe the answer is:
trepan
'saw' is the definition.
(I've seen this before)
'agent dressed in brown' is the wordplay.
'agent' becomes 'rep' (short for representative).
'dressed in' indicates putting letters inside (inserted letters are dressed in others).
'brown' becomes 'tan' (tan is a kind of brown).
'rep' placed inside 'tan' is 'TREPAN'.
(Other definitions for trepan that I've seen before include "Cranial surgical instrument" , "old saw" , "Perforate skull (med.)" , "cut through bone" , "Skull borer" .)