War is seen to produce fatigue (9)
I believe the answer is:
weariness
'fatigue' is the definition.
(feeling fatigued or weary)
'war is seen to produce' is the wordplay.
'to produce' is an anagram indicator.
'war'+'is'+'seen'='warisseen'
'warisseen' anagrammed gives 'WEARINESS'.
(Other definitions for weariness that I've seen before include "Lack of energy" , "Fatigue, tiredness" , "Tired feeling" , "State of fatigue or utter boredom" , "Lassitude" .)