Simple to be married before, in an old-fashioned way (4)
I believe the answer is:
mere
'simple' is the definition.
('mere' can be a synonym of 'simple')
'married before in an old-fashioned way' is the wordplay.
'married' becomes 'm'.
'before in an old-fashioned way' becomes 'ere' (archaic word for 'before').
'm'+'ere'='MERE'
'to be' acts as a link.
(Other definitions for mere that I've seen before include "Unimpressive" , "President Hollande's mother?" , "Previously pure" , "Lake - French mother" , "Pierre's mother identified" .)