Ornate four-poster, no end of usage, safe against certain decay (9)
I believe the answer is:
rustproof
'ornate four-poster no' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'end of usage safe against certain decay' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is located in the answer.
'decay' could be 'rust' (rusting is a kind of decaying) and 'rust' is found in the answer.
The remaining letters 'prof' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rustproof that I've seen before include "(Of iron) resistant to oxidisation" , "Impervious to irony tainting" , "(Of iron) not susceptible to corrosion" .)