Heavyweight introducing a posh car cover (7)
I believe the answer is:
tonneau
'car cover' is the definition.
(I've seen this in another clue)
'heavyweight introducing a posh' is the wordplay.
'heavyweight' becomes 'tonne' (I've seen this before).
'introducing' means one lot of letters go next to another (some letters go before others).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'tonne'+'a'+'u'='TONNEAU'
(Other definitions for tonneau that I've seen before include "Part of open car behind front seats" , "Rear of an open carriage" , "car's back seat" , "cover for motorist" , "Protective cover for open car" .)