Best to make a choice, one mother’s supporting (7)
I believe the answer is:
optimum
'best' is the definition.
('optimum' can be similar in meaning to 'best')
'make a choice one mother's supporting' is the wordplay.
'make a choice' becomes 'opt' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'mother' becomes 'mum' (mum is a kind of mother).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'opt'+'i'+'mum'='OPTIMUM'
'to' is the link.
(Other definitions for optimum that I've seen before include "Best (result)" , "The best possible" , "The most favourable condition" , "Best, most favourable" , "top condition" .)