Ready to change later (5)
I believe the answer is:
alert
'ready' is the definition.
(thesaurus)
'change later' is the wordplay.
'change' indicates anagramming the letters.
'later' with letters rearranged gives 'ALERT'.
'to' acts as a link.
(Other definitions for alert that I've seen before include "'Attentive, vigilant (5)'" , "'Attentive, watchful (5)'" , "Wide awake and watchful" , "Watchful and bright" , "Put on guard, warn" .)