Let everyone old get married (7)
I believe the answer is:
allowed
'let' is the definition.
(to allow something is to let it happen)
'everyone old get married' is the wordplay.
'everyone' becomes 'all' (all people).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'get married' becomes 'wed' (synonyms).
'all'+'o'+'wed'='ALLOWED'
(Other definitions for allowed that I've seen before include "Acknowledged" , "Owned" , "Gave permission" , "Permitted and granted" , "Given the chance" .)