Frank has a letter to write (4)
I believe the answer is:
open
'frank' is the definition.
('open' can be a synonym of 'frank')
'a letter to write' is the wordplay.
'a letter' becomes 'o'.
'to' says to put letters next to each other.
'write' becomes 'pen' ('pen' can be a synonym of 'write').
'o'+'pen'='OPEN'
'has' is the link.
(Other definitions for open that I've seen before include "Undecided" , "Candid; ajar" , "Admitting customers" , "sporting competition" , "Traditional room heater" .)