Wife that is most broad (6)
I believe the answer is:
widest
'most broad' is the definition.
('wide' can be a synonym of 'broad')
'wife that is' is the wordplay.
'wife' becomes 'w'.
'that is' becomes 'idest' ('id est' can be a synonym of 'that is').
'w'+'idest'='WIDEST'
(Other definitions for widest that I've seen before include "least slim" , "Most broad" , "spread maximally" , "most off target" , "showing maximal gap between sides" .)