Frank has a letter to write (4)
I believe the answer is:
open
'frank' is the definition.
(synonyms)
'a letter to write' is the wordplay.
'a letter' becomes 'o' ().
'to' means one lot of letters go next to another.
'write' becomes 'pen' (synonyms).
'o'+'pen'='OPEN'
'has' acts as a link.
(Other definitions for open that I've seen before include "Undecided" , "Candid; ajar" , "Admitting customers" , "sporting competition" , "Traditional room heater" .)