Some gung-ho ulcerous fiend (5)
I believe the answer is:
ghoul
'fiend' is the definition.
(I've seen this before)
'some gung-ho ulcerous' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'GHOUL' can be found hidden inside 'gung ho ulcerous'.
(Other definitions for ghoul that I've seen before include "shade" , "Morbid voyeur" , "Evil spirit or phantom" , "Gruesome fiend" , "Morbid soul" .)