From keg, Lee -- a pro -- loosened stopper (10)
I believe the answer is:
goalkeeper
'loosened stopper' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'from keg lee a pro' is the wordplay.
'from' indicates an anagram (from the letters).
'keg'+'lee'+'a'+'pro'='kegleeapro'
'kegleeapro' is an anagram of 'GOALKEEPER'.
Can you help me to learn more?
(Other definitions for goalkeeper that I've seen before include "Given, say" , "A Greek pole (anag.)" , "Seaman was one" , "Given one of these" .)