Shriek upsets pedestrians (6)
I believe the answer is:
hikers
'pedestrians' is the definition.
(hiker is a kind of pedestrian)
'shriek upsets' is the wordplay.
'upsets' is an anagram indicator.
'shriek' is an anagram of 'HIKERS'.
(Other definitions for hikers that I've seen before include "Foot travellers" , "People on foot" , "people on road" , "They walk the hills" , "Ramblers" .)