Truck's width no end of pain (5)
I believe the answer is:
wagon
'truck's' is the definition.
(I've seen this before)
'width no end of pain' is the wordplay.
'width' becomes 'w' (abbreviation in geometry).
'no end of' means to remove the last letter.
'pain' becomes 'agony' (agony is a kind of pain).
'agony' with its final letter taken away is 'agon'.
'w'+'agon'='WAGON'
(Other definitions for wagon that I've seen before include "Cart; rail truck" , "Vehicle for transporting goods" , "Van, lorry" , "Four-wheeled goods vehicle" , "Freight truck" .)