Tom Sawyer dismisses stray - one that sounds like a cat (4)
I believe the answer is:
meow
'tom sawyer dismisses stray one that sounds like a cat' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one that sounds like a cat' is the wordplay.
'one' becomes 'me' (a setter might use 'one' to humourously mean 'me').
'that sounds like a cat' becomes 'ow' (I am not sure about this - if you are sure you should believe this answer much more).
'me'+'ow'='MEOW'
Can you help me to learn more?
(Other definitions for meow that I've seen before include "Cry of a cat" , "Cat's cry" , "Tom's plea for attention" , "Beastly noise" , "Catty remark" .)