Veneer about to lose colour outside (6)
I believe the answer is:
facade
'veneer' is the definition.
(I've seen this in another clue)
'about to lose colour outside' is the wordplay.
'about' becomes 'ca' (circa).
'to lose colour' becomes 'fade' (I've seen this before).
'outside' indicates putting letters inside (some letters go outside others).
'ca' placed inside 'fade' is 'FACADE'.
(Other definitions for facade that I've seen before include "Front-of-house" , "Outer surfaces or face as of a building" , "Walton's setting of Sitwell's verses" , "False front" , "Building frontage, or sort of pretence" .)