One guaranteeing a key will be in a rusty state (6)
I believe the answer is:
surety
'one guaranteeing a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'key will be in a rusty state' is the wordplay.
I cannot quite understand how this works, but
'key' could be 'e' (a musical key) and 'e' is present in the answer.
an anagram of 'rusty' is 'surty' which is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for surety that I've seen before include "Safeguard; certainty" , "Guarantor" , "Legal safeguard against loss" , "One who takes responsibility for another's debts" , "Semitic language" .)