One may be dressed up to them - by numbers? (5)
I believe the answer is:
Here is my best explanation:
'numbers?' is the definition.
(nine is a kind of number)
I can't explain the rest of the clue.
(Other definitions for nines that I've seen before include "Dressed to these is to be perfectly turned out", "Dressed to these and you're well-turned-out", "squares", "In emergency, press three of these", "Numbers to which one may be dressed up?", "9s" and "Very smart if dressed to these".)