Mogul's sizeable toupee (6)
I believe the answer is:
bigwig
'mogul's' is the definition.
(I've seen this before)
'sizeable toupee' is the wordplay.
'sizeable' becomes 'big' (similar in meaning).
'toupee' becomes 'wig' (a toupee is a wig).
'big'+'wig'='BIGWIG'
(Other definitions for bigwig that I've seen before include "'Chief, top banana (6)'" , "VIP with large toupee" , "What may lie ahead largely" , "Somebody" , "Mogul" .)