Church altar pieces by an impostor (9)
I believe the answer is:
charlatan
'impostor' is the definition.
(I've seen this before)
'church altar pieces by an' is the wordplay.
'church' becomes 'ch' (abbreviation on maps etc).
'pieces' indicates anagramming the letters (the same pieces in a new order).
'by' is a charade indicator (letters next to each other).
'altar' anagrammed gives 'arlat'.
'ch'+'arlat'+'an'='CHARLATAN'
(Other definitions for charlatan that I've seen before include "Quack doctor" , "ake" , "Quack, fraud, a flamboyant deceiver" , "Impostor, quack" , "Deceiver who attracts customers with tricks or jokes" .)