Ready to alter, possibly (5)
I believe the answer is:
alert
'ready' is the definition.
(thesaurus)
'alter possibly' is the wordplay.
'possibly' indicates an anagram.
'alter' anagrammed gives 'ALERT'.
'to' acts as a link.
(Other definitions for alert that I've seen before include "Vigilant - warning" , "Watchfully awake" , "Wakeful, observant" , "Quick to notice" , "Danger warning" .)