Low river in high ground (4)
I believe the answer is:
moor
'high ground' is the definition.
'low river' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'river' becomes 'r'.
'moo'+'r'='MOOR'
'in' acts as a link.
(Other definitions for moor that I've seen before include "North African - secure a boat - tract of land" , "Heathery upland" , "Secure boat with ropes" , "Open land covered with heather and bracken" , "Othello, perhaps" .)