Furious buccaneer losing power (5)
I believe the answer is:
irate
'furious' is the definition.
(I know that furious can be written as irate)
'buccaneer losing power' is the wordplay.
'buccaneer' becomes 'pirate' (historical term for a pirate).
'losing' is a deletion indicator.
'power' becomes 'p' (symbol in physics).
'pirate' with 'p' taken away is 'IRATE'.
(Other definitions for irate that I've seen before include "Cross, irritated" , "Angry and incensed" , "I tear away, being annoyed" , "having got in wax" , "Spitting feathers" .)