Affliction seeming trivial to a blockhead (6)
I believe the answer is:
blight
'affliction' is the definition.
(blight is a kind of health problem)
'trivial to a blockhead' is the wordplay.
'trivial' becomes 'light' (light can mean trivial or casual).
'to a' is a charade indicator (letters next to each other) (I've seen this in other clues).
'blockhead' becomes 'b' (head letter of 'block').
'light' put after 'b' is 'BLIGHT'.
'seeming' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for blight that I've seen before include "Problem of plants or urban development" , "Potato disease" , "Pestilence or disease, as of potatoes" , "Withering disease (in plants)" , "Disease or fungus that withers plants, say potatoes" .)