Choose one for mother that's perfect (7)
I believe the answer is:
optimum
'perfect' is the definition.
(I've seen this in another clue)
'choose one for mother' is the wordplay.
'choose' becomes 'opt' (both can mean to pick something).
'one' becomes 'i' (Roman numeral).
'for' says to put letters next to each other.
'mother' becomes 'mum' (informal term for mother).
'opt'+'i'+'mum'='OPTIMUM'
'that's' is the link.
(Other definitions for optimum that I've seen before include "Most favourable amount" , "Most appropriate" , "Most favourable point" , "The best possible" , "Best, most favourable" .)