Pick one parent as the best (7)
I believe the answer is:
optimum
'the best' is the definition.
(optimum means the best possible)
'pick one parent' is the wordplay.
'pick' becomes 'opt' (as in opting for or picking something).
'one' becomes 'I' (Roman numeral).
'parent' becomes 'mum' (mum is a kind of parent).
'opt'+'i'+'mum'='OPTIMUM'
'as' is the link.
(Other definitions for optimum that I've seen before include "Most appropriate" , "Most favourable point" , "Most favourable (conditions)" , "Best, most favourable" , "Most favourable amount" .)