Vital English engineers in draft heading west (9)
I believe the answer is:
energised
'vital' is the definition.
(vital can mean full of life or energy)
'english engineers in draft heading west' is the wordplay.
'english' becomes 'E' (abbreviation as in OED).
'engineers' becomes 're' (Royal Engineers).
'in' is an insertion indicator.
'draft' becomes 'design' (both can mean to plan).
'heading west' shows that the letters should be reversed in order.
're' placed inside 'design' is 'desigren'.
'desigren' back-to-front is 'nergised'.
'e'+'nergised'='ENERGISED'
(Other definitions for energised that I've seen before include "Given drive" , "Roused" , "Animated" , "Gave vitality to" , "ready for action" .)