Newlywed needs to prepare for her partner (10)
I believe the answer is:
bridegroom
'her partner' is the definition.
(the bridegroom is the bride's partner)
'newlywed needs to prepare' is the wordplay.
'newlywed' becomes 'bride' (bride is a kind of newlywed).
'needs' is a charade indicator (letters next to each other).
'to prepare' becomes 'groom' ('groom' can be a synonym of 'prepare').
'bride'+'groom'='BRIDEGROOM'
'for' is the link.
(Other definitions for bridegroom that I've seen before include "new joiner" , "Union joiner" , "Man on his wedding day" , "new partner" .)