Insane letters that, on reflection, prove irksome (5)
I believe the answer is:
annoy
'prove irksome' is the definition.
I can't judge whether this defines the answer.
'insane letters that on reflection' is the wordplay.
'insane letters' becomes 'an' (I can't explain this - if you can you should believe this answer much more).
'that' becomes 'yon' (archaic term meaning 'that').
'on reflection' says the letters should be written backwards.
'yon' in reverse letter order is 'noy'.
'an'+'noy'='ANNOY'
Can you help me to learn more?
(Other definitions for annoy that I've seen before include "Nark - nettle" , "Bug, rile" , "Gall" , "eat" , "Upset" .)