Mother not drinking before daughter was important (8)
I believe the answer is:
mattered
'was important' is the definition.
(to matter is to be important)
'mother not drinking before daughter' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'not drinking' becomes 'tt' (short for teetotal).
'before' becomes 'ere' (usually literary).
'daughter' becomes 'd'.
'ma'+'tt'+'ere'+'d'='MATTERED'
(Other definitions for mattered that I've seen before include "Was of importance" , "Was significant" , "Counted, had weight" , "Counted, had significance" .)