Mother's the greatest! (7)
I believe the answer is:
maximum
'the greatest' is the definition.
(I've seen this before)
'mother's' is the wordplay.
I cannot really understand how this works, but
'mother' could be 'mum' (informal term for mother) and 'mum' is found within the answer.
The remaining letters 'maxi' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for maximum that I've seen before include "Upper limit" , "Top" , "Greatest possible quantity" , "Highest amount" , "Utmost" .)