Mystics baffled about energy affecting the whole body (8)
I believe the answer is:
systemic
'affecting the whole body' is the definition.
(I've seen this before)
'mystics baffled about energy' is the wordplay.
'baffled' indicates anagramming the letters.
'about' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'mystics' with letters rearranged gives 'systmic'.
'systmic' enclosing 'e' is 'SYSTEMIC'.
(Other definitions for systemic that I've seen before include "insecticide" , "Relating to the whole organism" , "City mess (anag)" , "Relating to a complex whole" , "Affecting a body, institution, etc as a whole" .)