Loose rocks by church make an unpleasant sound (7)
I believe the answer is:
Here is my best explanation:
'an unpleasant sound' is the definition.
'loose rocks by church' is the subsidiary indication.
'loose rocks' becomes 'scree'.
'by' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'make' is the link.
(Other definitions for screech that I've seen before include "Harsh, shrill cry", "Squeal", "Raucous sound", "cry of horror", "Sudden shrill cry", "Shriek", "Yell", "Loud harsh cry", "Piercing cry" and "Loud shrill cry".)