Behind spirit, phantom's head (4)
I believe the answer is:
rump
'behind' is the definition.
('rump' can be a synonym of 'behind')
'spirit phantom's head' is the wordplay.
'spirit' becomes 'rum' (I've seen this before).
'head' indicates taking the first letters.
The first letter of 'phantoms' is 'p'.
'rum'+'p'='RUMP'
(Other definitions for rump that I've seen before include "Mammal's hind part" , "Backside" , "Inferior remains; cut of steak" , "Meat (from behind?)" , "Hind part of body" .)