Smallest car mother wanted (7)
I believe the answer is:
minimum
'smallest' is the definition.
(similar in meaning)
'car mother wanted' is the wordplay.
I cannot really see how this works, but
'car' could be 'mini' (Mini Cooper) and 'mini' is located in the answer.
'mother' could be 'mum' (informal term for mother) and 'mum' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for minimum that I've seen before include "Lower limit" , "Lowest value" , "Lowest point" , "Least recorded value" , "Nadir" .)