United Nations starts to abandon boycott, leaving everyone powerless (6)
I believe the answer is:
unable
'powerless' is the definition.
('unable' can be similar in meaning to 'powerless')
'united nations starts to abandon boycott leaving everyone' is the wordplay.
'united nations' becomes 'un' ('UN' can be a synonym of 'united nations').
'starts to' says to take the initial letters.
The initial letters of 'abandon boycott leaving everyone' is 'able'.
'un'+'able'='UNABLE'
(Other definitions for unable that I've seen before include "Lacking the necessary skills" , "Incompetent sort of nebula" , "Not in a position (to)" , "Old, weak" , "Not having sufficient power" .)