A dance step that reflects vitality (3)
I believe the answer is:
pas
'a dance step' is the definition.
(I've seen this before)
'reflects vitality' is the wordplay.
'reflects' shows that the letters should be reversed in order.
'vitality' becomes 'sap' (I've seen this in another clue).
'sap' written backwards gives 'PAS'.
'that' is the link.
(Other definitions for pas that I've seen before include "Dad's" , "... de deux, ballet step for two" , "A bit of ballet" , "Office workers" , "Dance movement" .)