Choose a single mother as the best (7)
I believe the answer is:
optimum
'the best' is the definition.
(I've seen this before)
'choose a single mother' is the wordplay.
'choose' becomes 'opt' (I've seen this before).
'a single' becomes 'i' (one).
'mother' becomes 'mum' (mum is a kind of mother).
'opt'+'i'+'mum'='OPTIMUM'
'as' acts as a link.
(Other definitions for optimum that I've seen before include "The most favourable condition" , "model" , "Most favourable point" , "The best possible" , "top condition" .)