Car in ultimately unusable condition (6)
I believe the answer is:
estate
'car' is the definition.
(I've seen this before)
'ultimately unusable condition' is the wordplay.
'ultimately' indicates one should take the final letters.
'condition' becomes 'state' (I've seen this before).
The final letter of 'unusable' is 'e'.
'e'+'state'='ESTATE'
'in' acts as a link.
(Other definitions for estate that I've seen before include "All money and property owned at time of death" , "Piece of land with a large house or many small ones" , "Property - may go with rank" , "The whole of one's worldly goods" , "park perhaps" .)