On reflection, humour brings unpleasant fate (4)
I believe the answer is:
doom
'unpleasant fate' is the definition.
(doom is someone's downfall or death)
'on reflection humour' is the wordplay.
'on reflection' shows that the letters should be reversed in order.
'humour' becomes 'mood' (I've seen this before).
'mood' reversed gives 'DOOM'.
'brings' is the link.
(Other definitions for doom that I've seen before include "Terrible fate; death" , "Mood of grim fate" , "Downfall" , "In disaster" , "Day of reckoning" .)