Important person with large hairpiece (6)
I believe the answer is:
bigwig
'important person' is the definition.
(bigwig is a kind of important person)
'with large hairpiece' is the wordplay.
'with' is a charade indicator (letters next to each other).
'large' becomes 'big' (synonyms).
'hairpiece' becomes 'wig' (wig is a kind of hairpiece).
'big' next to 'wig' is 'BIGWIG'.
(Other definitions for bigwig that I've seen before include "Luminary" , "Dignitary (sl.)" , "VIP with large toupee" , "Important person (colloq.)" , "Mogul" .)