Alert to one evil possibility (5)
I believe the answer is:
alive
'alert' is the definition.
(I know that alert can be written as alive)
'one evil possibility' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'possibility' is an anagram indicator (another possible arrangement of the letters).
'evil' anagrammed gives 'live'.
'a'+'live'='ALIVE'
'to' acts as a link.
(Other definitions for alive that I've seen before include "Existing - vigorous" , "Still with us" , "Not yet pushing up the daisies" , "Go or walk briskly" , "Active - living" .)