Thing to alter between today and tomorrow? (7)

I believe the answer is:

tonight

I'm a little stuck... Click here to teach me more about this clue!

'today and tomorrow?' is the definition.
I can't tell whether this definition defines the answer.

'thing to alter' is the wordplay.
'alter' indicates an anagram.
'thing'+'to'='thingto'
'thingto' with letters rearranged gives 'TONIGHT'.

'between' acts as a link.

Can you help me to learn more?

(Other definitions for tonight that I've seen before include "On tight (anag.)" , "Later this evening" , "Later today" , "Got thin (anag)" , "the coming time for retirement" .)

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.