Short dresses coming up -- very shortly?
I believe the answer is:
'short dresses coming' is the definition.
Both the definition and answer are adverbs. Perhaps you can see an association between them that I can't see?
'up very shortly?' is the wordplay.
'up' becomes 'ton' (I am not sure about this - if you are sure you should believe this answer much more).
'very' becomes 'light'.
'shortly?' means to remove the first letter (I've seen 'shortly' mean this).
'light' with its first letter taken away is 'ight'.
Can you help me to learn more?
(Other definitions for tonight that I've seen before include "the coming time for retirement" , "Dark time ahead" , "next retirement occasion" , "West Side Story number" , "What succeeds this afternoon" .)