Energy field (4)
I believe the answer is:
aura
'energy field' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aura that I've seen before include "'Air, atmosphere (4)'" , "Invisible emanation" , "Intangible essence emanating from a person" , "Nimbus" , "Distinctive quality around a person or place" .)