One with a fatal vulnerability longs to take sick (8)
I believe the answer is:
achilles
'one with a fatal vulnerability' is the definition.
I can't judge whether this defines the answer.
'longs to take sick' is the wordplay.
'longs' becomes 'aches' ('ache' can be a synonym of 'long').
'to take' indicates putting letters inside (inserted letters are taken in).
'sick' becomes 'ill' (similar in meaning).
'aches' placed around 'ill' is 'ACHILLES'.
Can you help me to learn more?
(Other definitions for achilles that I've seen before include "great hero" , "Legendary Greek with injured heel" , "Ancient Greek hero" , "Great Greek with weak heel" , "Greek hero who gave his name to a bad heel" .)