Thing to change into this evening (7)
I believe the answer is:
tonight
'this evening' is the definition.
('tonight' can be a synonym of 'this evening')
'thing to change' is the wordplay.
'change' indicates an anagram.
'thing'+'to'='thingto'
'thingto' is an anagram of 'TONIGHT'.
'into' is the link.
(Other definitions for tonight that I've seen before include "Later this evening" , "next retirement occasion" , "What succeeds this afternoon" , "Hotting up for after sundown" , "On tight (anag.)" .)