Exhausted American joins European power-sharing party (4,2)
I believe the answer is:
used up
'exhausted' is the definition.
(similar in meaning)
'american joins european power-sharing party' is the wordplay.
'american' becomes 'US' (abbreviation for United States).
'joins' is a charade indicator (letters next to each other).
'european' becomes 'E' (abbreviation e.g. EU).
'power-sharing party' becomes 'DUP' (Democratic Unionist Party of Northern Ireland).
'us'+'e'+'dup'='USED-UP'
(Other definitions for used up that I've seen before include "Exhausted the supply" , "Spent" , "Consumed completely" , "Consumed, finished" .)