Powerless (8)
'powerless' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for impotent that I've seen before include "Lacking power" , "Helpless, powerless" , "Ineffective" , "Mon petit may be powerless" , "Useless" .)
'powerless' is the definition.
(similar in meaning)
This is the entire clue.
(Other definitions for unable that I've seen before include "Not having the neccessary means" , "Old, weak" , "Incompetent or powerless" , "Lacking the necessary skills" , "Lacking power (to do something)" .)
(Other definitions for over a barrel that I've seen before include "In a mess" , "Unable to manoeuvre" , "forty gallons or so?" , "forty-odd gallons of beer?" .)