Frank is old writer (4)
I believe the answer is:
open
'frank' is the definition.
(synonyms)
'old writer' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'writer' becomes 'pen' (something which writes).
'o'+'pen'='OPEN'
'is' acts as a link.
(Other definitions for open that I've seen before include "sporting competition" , "Not shut or secret" , "Golf tournament" , "Liberal" , "William --, English poet" .)