A yield over 50 is providing a superior capital position (9)
I believe the answer is:
acropolis
'a superior capital position' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'a yield over 50 is' is the wordplay.
'yield' becomes 'crop' (yield can mean a crop or harvest).
'over 50' becomes 'ol' (I can't justify this - if you can you should believe this answer much more).
'a'+'crop'+'ol'+'is'='ACROPOLIS'
'providing' is the link.
Can you help me to learn more?
(Other definitions for acropolis that I've seen before include "Upper fortified part of Greek city - Athens's is famous" , "high point" , "Parthenon location" , "Greek tourist spot" , "old citadel" .)