Man dressed as Celt (6)
I believe the answer is:
castle
'man' is the definition.
('man' can mean a chess piece)
'dressed as celt' is the wordplay.
'dressed' indicates anagramming the letters ('dress' can mean 'prepare').
'as'+'celt'='ascelt'
'ascelt' anagrammed gives 'CASTLE'.
(Other definitions for castle that I've seen before include "Fort" , "Fortified mansion" , "Fortified stronghold" , "ancient building?" , "Dover, perhaps" .)