Not as high (5)
I believe the answer is:
lower
'not as high' is the definition.
('low' can be the opposite of 'high' so perhaps 'lower' is reasonable for 'not as high')
This is the entire clue.
(Other definitions for lower that I've seen before include "of lesser status" , "Let down; frown" , "where are trousers in relation?" , "Cow" , "put down" .)