Shivering fit: ague causes weariness (7)
I believe the answer is:
fatigue
'weariness' is the definition.
(synonyms)
'shivering fit ague' is the wordplay.
'shivering' indicates an anagram.
'fit'+'ague'='fitague'
'fitague' is an anagram of 'FATIGUE'.
'causes' acts as a link.
(Other definitions for fatigue that I've seen before include "Tommy's punishment" , "Tire" , "military punishment" , "Tiredness after effort" , "military chore" .)