Attempted when 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' anagrammed gives 'TRIED'.
'when' is the link.
(Other definitions for tried that I've seen before include "Endeavoured" , "Made an attempt" , "Annoyed when obliged to accept a rule" , "Tested" , "Had a crack" .)