Asian river some find useful (5)
I believe the answer is:
indus
'asian river' is the definition.
(the Indus is an Asian river)
'some find useful' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'INDUS' can be found hidden inside 'find useful'.
(Other definitions for indus that I've seen before include "South Asian river" , "Long river of Asia" , "Long Asian river" , "River flowing from Tibet to the Arabian Sea" , "running water" .)