Furious, freebooter having lost power (5)
I believe the answer is:
irate
'furious' is the definition.
(thesaurus)
'freebooter having lost power' is the wordplay.
'freebooter' becomes 'pirate' (pirate is a kind of freebooter).
'having lost' is a deletion indicator.
'power' becomes 'p'.
'pirate' with 'p' taken away is 'IRATE'.
(Other definitions for irate that I've seen before include "Spitting feathers" , "raging" , "ranting" , "Ticked off" , "Fuming" .)