Old black cat protecting four in forgotten state (8)
I believe the answer is:
oblivion
'forgotten state' is the definition.
(I've seen this in another clue)
'old black cat protecting four' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'black' becomes 'b' (abbreviation as in pencil classifications).
'cat' becomes 'lion' (lion is a kind of cat).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'four' becomes 'IV' (Roman numerals).
'lion' enclosing 'iv' is 'livion'.
'o'+'b'+'livion'='OBLIVION'
'in' is the link.
(Other definitions for oblivion that I've seen before include "State of being forgotten and disregarded" , "State of being altogether forgotten" , "State of total unconciousness" , "State of total forgetfulness or unconsciousness" , "Neglected state" .)