Slowing down after abnormal power and vigour (6)
I believe the answer is:
esprit
'vigour' is the definition.
'slowing down after abnormal power' is the wordplay.
'slowing down' becomes 'rit' (I've seen this in other clues).
'after' means one lot of letters go next to another.
'abnormal power' becomes 'esp' (extrasensory perception).
'rit' after 'esp' is 'ESPRIT'.
'and' is the link.
(Other definitions for esprit that I've seen before include "Sparkling quality" , "Stripe (anag)" , "What good corps needs" , "Lively and vivacious quality" , "Sprightliness" .)