Firm support for driver's seat (6)
I believe the answer is:
settee
'seat' is the definition.
(I've seen this before)
'firm support for driver's' is the wordplay.
'firm' becomes 'set' (I've seen this before).
'support for driver' becomes 'tee' (driver is a type of golf club).
'set'+'tee'='SETTEE'
(Other definitions for settee that I've seen before include "Long upholstered seat with a back" , "Item of furniture" , "Chesterfield" , "Small sofa" , "chair" .)