Attempted, terribly tired (5)
I believe the answer is:
tried
'attempted' is the definition.
('try' can be a synonym of 'attempt')
'terribly tired' is the wordplay.
'terribly' is an anagram indicator.
'tired' is an anagram of 'TRIED'.
(Other definitions for tried that I've seen before include "Essayed" , "Annoyed when obliged to accept a rule" , "Proven by experience" , "Made an effort" , "checked out" .)