Shivering fit, ague, causing weariness (7)
I believe the answer is:
fatigue
'weariness' is the definition.
(I know that weariness can be written as fatigue)
'shivering fit ague' is the wordplay.
'shivering' indicates anagramming the letters.
'fit'+'ague'='fitague'
'fitague' is an anagram of 'FATIGUE'.
'causing' is the link.
(Other definitions for fatigue that I've seen before include "military chore" , "Tire" , "Weariness, exhaustion" , "weary" , "Temporary loss of strength and energy" .)