Possibly alert to change (5)
I believe the answer is:
alter
'to change' is the definition.
(synonyms)
'possibly alert' is the wordplay.
'possibly' indicates an anagram.
'alert' with letters rearranged gives 'ALTER'.
(Other definitions for alter that I've seen before include "Change, make different" , "convert" , "Vary" , "Become different" , "and 10 Across: Second self" .)