Tom's nightwear? Perfect! (3,4,7)
I believe the answer is:
the cats pyjamas
'perfect' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'tom's nightwear?' is the wordplay.
I cannot really understand how this works, but
'tom's' could be 'cats' (a tom is a male cat) and 'cats' is present in the answer.
'nightwear?' could be 'pyjamas' (pyjamas is a kind of nightwear) and 'pyjamas' is present in the answer.
The remaining letters 'the' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for the cats pyjamas that I've seen before include "as worn by queen at night?" , "The ideal thing (dated colloq.)" , "The business" , "that Tom's in prior to retiring?" , "A fantastic thing" .)