Legendary figure bringing about illness, suffering and the onset of barbarism (7)
I believe the answer is:
beowulf
'legendary figure' is the definition.
The definition suggests a singular noun which matches the answer.
'bringing about illness suffering and the onset of barbarism' is the wordplay.
'bringing about' shows that the letters should be reversed in order (I've seen 'brought about' mean this).
'illness' becomes 'flu' (flu is a kind of health problem).
'suffering' becomes 'woe' (synonyms).
'and' says to put letters next to each other.
'the onset of barbarism' becomes 'b' (I can't explain this - if you can you should give a lot more credence to this answer).
'flu'+'woe'='fluwoe'
'fluwoe' back-to-front is 'eowulf'.
'eowulf' after 'b' is 'BEOWULF'.
Can you help me to learn more?
(Other definitions for beowulf that I've seen before include "Anglo-Saxon epic poem" , "Old English epic poem" , "An old tale" , "[SCAN-DIN-AVIAN] poem" , "Germanic hero" .)