Endless worry: that's one's motor! (3)
I believe the answer is:
car
'one's motor' is the definition.
'car' can be an answer for 'motor' (informal term for car). I'm unsure of the 'one's' bit.
'endless worry' is the wordplay.
'endless' means to remove the last letter.
'worry' becomes 'care' (eg without a care means without a worry).
'care' with its last letter taken off is 'CAR'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for car that I've seen before include "Four-wheeled vehicle" , "Motor vehicle" , "saloon perhaps" , "Private vehicle" , "Wheels" .)