Screechy birds regularly encountered in southern river (8)
I believe the answer is:
strident
'screechy' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'birds regularly encountered in southern river' is the wordplay.
'regularly encountered' indicates alternate letters.
'in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'river' becomes 'trent' (River Trent).
The alternating letters of 'birds' are 'id'.
's'+'trent'='strent'
'id' put within 'strent' is 'STRIDENT'.
Can you help me to learn more?
(Other definitions for strident that I've seen before include "Loud and harsh over tridents" , "(Of a voice) loud and grating" , "Grating - urgent" , "Roaring" , "Harsh, grating" .)