Stop daughter leaving new partner for good (9)
I believe the answer is:
eternally
'for good' is the definition.
(I've seen this before)
'stop daughter leaving new partner' is the wordplay.
I cannot quite see how this works, but
'new' could be 'n' and 'n' is found in the answer.
'partner' could be 'ally' (I've seen this in other clues) and 'ally' is present in the answer.
This explanation may well be incorrect...
(Other definitions for eternally that I've seen before include "For evermore" , "Always and ever" , "all the time" , "for good" , "constantly" .)