Frank has to ring a writer (4)
I believe the answer is:
open
'frank' is the definition.
(synonyms)
'ring a writer' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'a writer' becomes 'pen' (something which writes).
'o'+'pen'='OPEN'
'has to' acts as a link.
(Other definitions for open that I've seen before include "Contest that includes all comers" , "Exposed; free" , "Not closed or obstructed" , "Sports contest" , "unsealed" .)