Low river berth (4)
I believe the answer is:
moor
'berth' is the definition.
(I've seen this before)
'low river' is the wordplay.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'river' becomes 'r'.
'moo'+'r'='MOOR'
(Other definitions for moor that I've seen before include "Ilkley or Bodmin" , "Wild land; tie up" , "Heathery upland" , "Land mainly high" , "Berth, tie up" .)