Supporter has key number (4)
I believe the answer is:
bone
'supporter' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'key number' is the wordplay.
'key' becomes 'b' (eg B major, musical key).
'number' becomes 'one' (one is a kind of number).
'b'+'one'='BONE'
'has' is the link.
Can you help me to learn more?
(Other definitions for bone that I've seen before include "Scapula, radius or tibia?" , "Possibly hammer" , "Skeleton material" , "See 6" , "Ossified material" .)