Box car past its best (5)

I believe the answer is:

crate

I believe this is a double definition.

'box' is the first definition.
(crate is a kind of box)

'car past its best' is the second definition.
(informal term for an old car)

(Other definitions for crate that I've seen before include "old banger" , "Trace to a large container" , "Carton; old plane" , "Packing case with wooden slats" , "Clapped-out vehicle" .)

I've seen this clue in The Telegraph.

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