Average is low (4)
I believe the answer is:
mean
This clue is a double definition.
'average' is the first definition.
(type of average)
'low' is the second definition.
(both can mean nasty)
'is' acts as a link.
(Other definitions for mean that I've seen before include "The average could be miserly" , "Intend; not generous" , "Ungenerous" , "Tight - average" , "cause" .)