Not high (3)
I believe the answer is:
low
'not high' is the definition.
('low' is the opposite to 'high')
This is the entire clue.
(Other definitions for low that I've seen before include "Unimportant sort of owl" , "Opposite of high" , "Deep sound of cattle - not high" , "Not high or loud" , "something from Guernsey?" .)