Terribly alert at some time in the future (5)
I believe the answer is:
later
'at some time in the future' is the definition.
(synonyms)
'terribly alert' is the wordplay.
'terribly' indicates anagramming the letters.
'alert' anagrammed gives 'LATER'.
(Other definitions for later that I've seen before include "Alter (anag.)" , "More recent" , "at a subsequent point" , "At a subsequent time" , "at a future date" .)