Gloss surplus, treating fences (6)
I believe the answer is:
lustre
'gloss' is the definition.
(I've seen this before)
'surplus treating fences' is the wordplay.
'fences' indicates the answer is hidden within the clue (the solution is fenced in by the clue).
'LUSTRE' is hidden within 'surplus treating'.
(Other definitions for lustre that I've seen before include "Glory, distinction" , "Radiance; reflective surface" , "Distinction < in five years" , "Gloss, brightness" , "Prestige" .)