Drained river? Bound to go across it (5)
I believe the answer is:
tired
'drained' is the definition.
(tiring is a kind of draining)
'river? bound to go across it' is the wordplay.
'river?' becomes 'r' (abbreviation for river).
'bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'to go across it' indicates putting letters inside (some letters go around or across others).
'r' placed into 'tied' is 'TIRED'.
(Other definitions for tired that I've seen before include "Drowsy" , "Worn out" , "Depleted of energy" , "Worn-out" , "Weary, jaded" .)