To be frank, there's nothing to write (4)
I believe the answer is:
open
'to be frank' is the definition.
('open' can be a synonym of 'frank')
'nothing to write' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to' means one lot of letters go next to another.
'write' becomes 'pen' (synonyms).
'o'+'pen'='OPEN'
'there's' is the link.
(Other definitions for open that I've seen before include "Frank - accessible" , "Widely known" , "Frank; unfold" , "Uncovered" , "Accessible - candid" .)