For beneficial result, put key on old possession (7,6)
I believe the answer is:
placebo effect
'possession' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'for beneficial result put key on old' is the wordplay.
I cannot really see how this works, but
'result' could be 'effect' (synonyms) and 'effect' is found within the answer.
'put' could be 'place' (place can mean to put something somewhere) and 'place' is found within the answer.
'key' could be 'b' (eg B major, musical key) and 'b' is present in the answer.
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for placebo effect that I've seen before include "Psychological action of inert drug" , "psychological remedy" , "Inactive drug's result relying on the patient's belief in its worth" .)