Subdues mother's sisters beneath daughter (6)
I believe the answer is:
daunts
'subdues' is the definition.
(to daunt is to intimidate or subdue)
'mother's sisters beneath daughter' is the wordplay.
'mother's sisters' becomes 'aunts'.
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'daughter' becomes 'd' (genealogical abbreviation).
'aunts' after 'd' is 'DAUNTS'.
(Other definitions for daunts that I've seen before include "Frightens or discourages" , "Makes afraid or discouraged" , "Cows" , "intimidates" .)