Tom's nightwear - exactly right! (3,4,7)
I believe the answer is:
the cats pyjamas
'exactly right' 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 quite see how this works, but
'tom's' could be 'hecats' (tom can mean a male cat) and 'hecats' is found in the answer.
'nightwear' could be 'pyjamas' (pyjamas is a kind of nightwear) and 'pyjamas' is found in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for the cats pyjamas that I've seen before include "The business" , "as worn by queen at night?" , "that Tom's in prior to retiring?" , "The ideal thing (dated colloq.)" , "A fantastic thing" .)