Vital life force possessed by Achilles (3)
I believe the answer is:
chi
'vital life force' is the definition.
(I've seen this in another clue)
'possessed by achilles' is the wordplay.
'possessed by' says the answer is hidden in the clue.
'CHI' is hidden in the letters of 'achilles'.
(Other definitions for chi that I've seen before include "character from Euripides?" , "Classical character" , "The life force, according to traditional Chinese medicine" , "Greek character" , "In Chinese medicine, the life force inherent in all things" .)