Attain a river, somehow (6)
I believe the answer is:
arrive
'attain' is the definition.
(I've seen this in another clue)
'a river somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'river' with letters rearranged gives 'rrive'.
'a'+'rrive'='ARRIVE'
(Other definitions for arrive that I've seen before include "Reach destination or succeed" , "emerge" , "Make it" , "Reach destination or achieve success" , "Drop in" .)