Nothing to write? That's clear (4)
I believe the answer is:
open
'clear' is the definition.
('open' can be a synonym of 'clear')
'nothing to write?' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'to write?' becomes 'pen' ('pen' can be a synonym of 'write').
'o'+'pen'='OPEN'
'that's' is the link.
(Other definitions for open that I've seen before include "golf contest" , "Willing" , "Liberal" , "Candid and undisguised" , "Not shut or secret" .)