Best choice, lost on mother (7)
I believe the answer is:
optimum
'best' is the definition.
(optimum means the best possible)
'choice lost on mother' is the wordplay.
'choice' becomes 'option' (synonyms).
'lost' suggests deleting specific letters.
'mother' becomes 'mum' (informal term for mother).
'option' with 'on' taken out is 'opti'.
'opti'+'mum'='OPTIMUM'
(Other definitions for optimum that I've seen before include "The most favourable condition" , "model" , "Best (result)" , "Best, most favourable" , "Most favourable amount" .)