Glimpse one impersonating copper (6)
I believe the answer is:
apercu
'glimpse' is the definition.
(I've seen this in another clue)
'one impersonating copper' is the wordplay.
'one impersonating' becomes 'aper' (I am not sure about this - if you are sure you should believe this answer much more).
'copper' becomes 'Cu' (chemical symbol).
'aper'+'cu'='APERCU'
Can you help me to learn more?
(Other definitions for apercu that I've seen before include "Illuminating or entertaining point (from French)" , "Illuminating comment or brief reference, to the French" , "summary, insight; race up (anag.)" , "Witty remark" , "Brief sketch, outline" .)