Paint, a gallon a write-off (5)
I believe the answer is:
gloss
'paint' is the definition.
(I have seen 'Shiny paint ' mean 'gloss' so perhaps 'paint' could also mean 'gloss')
'a gallon a write-off' is the wordplay.
'a gallon' becomes 'g' (abbreviation for the unit of liquid).
'a write-off' becomes 'loss' (financial loss).
'g'+'loss'='GLOSS'
(Other definitions for gloss that I've seen before include "marginal note" , "Sheen - paint" , "Shiny surface" , "Render plausible" , "phrase's explanation" .)