King put on upper-class, grand wig (3)
I believe the answer is:
rug
'wig' is the definition.
(rug is a slang term for a wig)
'king put on upper-class grand' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'put on' is a charade indicator (letters next to each other).
'upper-class' becomes 'u' (similar in meaning).
'grand' becomes 'g' (G is short for grand as in a thousand).
'r'+'u'+'g'='RUG'
(Other definitions for rug that I've seen before include "grace the boards" , "Thick mat" , "Wig (facetious)" , "Informal term for a toupee" , "wrap" .)