Invalid point papal decree initially rejected (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
'point papal decree initially rejected' is the wordplay.
'point' becomes 'n' (North).
'papal decree' becomes 'bull' (I've seen this in another clue).
'initially rejected' means to remove the first letter.
'bull' with its initial letter removed is 'ull'.
'n'+'ull'='NULL'
(Other definitions for null that I've seen before include "Without value or consequence" , "Invalid; empty" , "Having the value zero -- void" , "Invalid - not binding" , "Non-existent and void" .)