Nothing to write its plain to see (4)
I believe the answer is:
open
'its plain to see' is the definition.
I can't tell whether this defines the answer.
'nothing to write' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to' says to put letters next to each other.
'write' becomes 'pen' (synonyms).
'o'+'pen'='OPEN'
Can you help me to learn more?
(Other definitions for open that I've seen before include "Not yet decided - to start" , "Allowing access" , "Frank - accessible" , "'Frank, not secret (4)'" , "Plain" .)