Best imaginable mother? (7)
I believe the answer is:
optimum
'best' is the definition.
(optimum means the best possible)
'imaginable mother?' is the wordplay.
I cannot quite understand how this works, but
'mother?' could be 'mum' (informal term for mother) and 'mum' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for optimum that I've seen before include "Most appropriate" , "model" , "The most favourable condition" , "Most favourable point" , "Best, most favourable" .)