Only married before (4)
I believe the answer is:
mere
'only' is the definition.
(I've seen this before)
'married before' is the wordplay.
'married' becomes 'm' (abbreviation, e.g. in genealogy).
'before' becomes 'ere' (poetic term for 'before').
'm'+'ere'='MERE'
(Other definitions for mere that I've seen before include "Previously pure" , "Small area of standing water" , "Lake; simple" , "Without additions or modifications" , "President Hollande's mother?" .)