Vital force only somewhat achievable (3)
I believe the answer is:
chi
'vital force' is the definition.
(life force in Chinese philosophy)
'only somewhat achievable' is the wordplay.
'only somewhat' says the answer is hidden in the clue (only some of the letters in the clue).
'CHI' can be found hidden inside 'achievable'.
(Other definitions for chi that I've seen before include "One character abroad" , "character from Euripides?" , "T'ai -, martial art" , "seen in Greek writing" , "Letter written by European" .)