Philosopher is alert to changes (9)
I believe the answer is:
aristotle
'philosopher' is the definition.
(Aristotle is an example)
'is alert to changes' is the wordplay.
'changes' is an anagram indicator.
'is'+'alert'+'to'='isalertto'
'isalertto' is an anagram of 'ARISTOTLE'.
(Other definitions for aristotle that I've seen before include "Peripatetic teacher" , "Ancient Greek philosopher" , "Philosopher - treats oil (anag)" , "Realist to be like great Greek philosopher" , "Thinking Greek" .)