Lightweight without substance (6)
I believe the answer is:
nobody
'lightweight' is the definition.
(I've seen this before)
'without substance' is the wordplay.
'without' becomes 'no'.
'substance' becomes 'body' (body substance is a kind of substance).
'no'+'body'='NOBODY'
(Other definitions for nobody that I've seen before include "Not one person" , "Inadequacy of wine" , "Lightweight" , "Nonentity" , "being without value" .)