Assist first-class daughter (3)
I believe the answer is:
aid
'assist' is the definition.
(synonyms)
'first-class daughter' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'daughter' becomes 'd' (genealogical abbreviation).
'ai'+'d'='AID'
(Other definitions for aid that I've seen before include "Relief" , "A hand" , "welfare" , "encouragement" , "Good point" .)