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