To be candid, there's nothing to write (4)
I believe the answer is:
open
'to be candid' is the definition.
(I've seen this before)
'nothing to write' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to' is a charade indicator (letters next to each other).
'write' becomes 'pen' (synonyms).
'o'+'pen'='OPEN'
'there's' is the link.
(Other definitions for open that I've seen before include "Agape" , "Undo; ready for business" , "Allowing entry and exit" , "Frank; unobstructed" , "Sports contest" .)