Drier river taking last of water (5)
I believe the answer is:
airer
'drier' is the definition.
(I've seen this before)
'river taking last of water' is the wordplay.
'river' becomes 'aire' (I've seen this before**).
'taking' says to put letters next to each other.
'last of' suggests the final letters.
The last letter of 'water' is 'r'.
'aire'+'r'='AIRER'
(Other definitions for airer that I've seen before include "Clothes-horse" , "Support for drying" , "Clothes-drying frame" , "Frame for drying clothes" , "Clothes drier" .)