Soon, a new and old measure (4)
I believe the answer is:
anon
'soon' is the definition.
(synonyms)
'a new and old measure' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'and' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'measure' becomes 'n' (N is an example).
'a'+'n'+'o'+'n'='ANON'
(Other definitions for anon that I've seen before include "coming after a call" , "Not identified by name" , "before long" , "Soon; unnamed" , "Incognito (abbrev)" .)