Tom Tom the musical. . . (4)
I believe the answer is:
cats
'the musical' is the definition.
(Andrew Lloyd Webber musical)
'tom tom' is the wordplay.
'tom' becomes 'cat' (I have seen 'A tom ' mean 'cat' so perhaps 'tom' could also mean 'cat').
'tom' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'cat'+'s'='CATS'
Can you help me to learn more?
(Other definitions for cats that I've seen before include "Popular musical" , "Jaguars, say" , "Musical; jazz fans" , "Musical from TS Eliot" , "Show" .)