Choose one parent as the best (7)
I believe the answer is:
optimum
'the best' is the definition.
(optimum means the best possible)
'choose one parent' is the wordplay.
'choose' becomes 'opt' (both can mean to pick something).
'one' becomes 'i' (Roman numeral).
'parent' becomes 'mum' (mum means a mother).
'opt'+'i'+'mum'='OPTIMUM'
'as' acts as a link.
(Other definitions for optimum that I've seen before include "Best (result)" , "Most appropriate" , "top condition" , "The best possible" , "Most favourable point" .)