Jalopy's constant speed (5)
I believe the answer is:
crate
'jalopy's' is the definition.
(I've seen this before)
'constant speed' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'speed' becomes 'rate' (both can mean pace).
'c'+'rate'='CRATE'
(Other definitions for crate that I've seen before include "Clapped-out vehicle" , "Jalopy" , "Packing container" , "Carton; old plane" , "Dilapidated vehicle" .)