Stallone keeps up power source (6)
I believe the answer is:
supply
'source' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'stallone keeps up power' is the wordplay.
'stallone' becomes 'sly' (nickname for Sylvester Stallone).
'keeps' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'up'+'p'='upp'
'sly' placed around 'upp' is 'SUPPLY'.
Can you help me to learn more?
(Other definitions for supply that I've seen before include "in a compliant manner" , "Lay on" , "Give" , "Stand-in teacher" , "with considerable flexibility" .)