Great weight? Gold in detachable car cover? (7)
I believe the answer is:
tonneau
'car cover?' is the definition.
(I've seen this before)
'great weight? gold' is the wordplay.
'great weight?' becomes 'tonne' (I've seen this before).
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'tonne'+'au'='TONNEAU'
'in detachable' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tonneau that I've seen before include "car's back seat" , "cover for motorist" , "vintage car part" , "Seat cover" , "Part of open car behind front seats" .)