Constant speed in old car (5)
I believe the answer is:
crate
'old car' is the definition.
(informal term for an old car)
'constant speed' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'speed' becomes 'rate' (the speed at which something happens).
'c'+'rate'='CRATE'
'in' acts as a link.
(Other definitions for crate that I've seen before include "Clapped-out vehicle" , "Wooden container" , "Old car" , "Dilapidated vehicle" , "Packing-case made of wooden slats" .)