Peaty land low on river (4)
I believe the answer is:
moor
'peaty land' is the definition.
(I've seen this in another clue)
'low on river' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'river' becomes 'r' (abbreviation for river).
'moo'+'r'='MOOR'
(Other definitions for moor that I've seen before include "Upland tract - eg Othello" , "Upland tract - secure boat" , "Fasten" , "Make fast a boat - open tract of rough ground" , "North African - secure a boat - tract of land" .)