Bound to admit last bit of water's drained (5)
I believe the answer is:
tired
'drained' is the definition.
(tiring is a kind of draining)
'bound to admit last bit of water's' is the wordplay.
'bound' becomes 'tied' ('tie' can be a synonym of 'bind').
'to admit' indicates putting letters inside.
'last bit of' says to take the final letters.
The final letter of 'water' is 'r'.
'tied' going around 'r' is 'TIRED'.
(Other definitions for tired that I've seen before include "In need of sleep" , "Drowsy" , "In need of rest" , "Spent" , "Worn out" .)