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