Key rent is a steady supply (4)
I believe the answer is:
drip
'a steady supply' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'key rent' is the wordplay.
'key' becomes 'd' (musical key).
'rent' becomes 'rip' (rent can mean a tear in something).
'd'+'rip'='DRIP'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for drip that I've seen before include "Fall in drops - weedy bloke" , "Fall or let fall in drops" , "Fall in droplets -- weedy person" , "Fall from a leaking tap - weakling" , "Leak from tap" .)