Tom's bedroom attire? It's the best thing going (3,4,7)
I believe the answer is:
the cats pyjamas
I believe this clue is a double definition.
'tom's bedroom attire?' is the first definition.
(tom is a male cat)
'the best thing going' is the second definition.
(the cat's pyjamas can mean something excellent)
'it's' acts as a link.
(Other definitions for the cats pyjamas that I've seen before include "The business" , "A fantastic thing" , "that Tom's in prior to retiring?" , "as worn by queen at night?" , "Real gem" .)