Freebooter losing head, furious (5)
I believe the answer is:
irate
'furious' is the definition.
(I know that irate can be written as irate)
'freebooter losing head' is the wordplay.
'freebooter' becomes 'pirate' (pirate is a kind of freebooter).
'losing head' means to remove the first letter.
'pirate' with its first letter taken off is 'IRATE'.
(Other definitions for irate that I've seen before include "Angry, enraged" , "A tier (anag)" , "Indignant, angry" , "Mad" , "Narked" .)