The young Mr Pitt (4)
I believe the answer is:
brad
'pitt' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see a link between them that I don't see?
'the young mr' is the wordplay.
'the young' becomes 'br' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mr' becomes 'Ad'.
'br'+'ad'='BRAD'
Can you help me to learn more?
(Other definitions for brad that I've seen before include "One may secure" , "Type of nail" , "One to get hammered" , "-- Pitt, US actor" , "Fixer" .)