Bad-tempered beast devouring ram's head (6)
I believe the answer is:
morose
'bad-tempered' is the definition.
'beast devouring ram's head' is the wordplay.
'beast' becomes 'moose'.
'devouring' is an insertion indicator.
'head' suggests taking the first letters.
The initial letter of 'rams' is 'r'.
'moose' placed around 'r' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Sullen and brooding" , "Miserable" , "in gloomy mood" , "blue" , "Silent, gloomy and ill-tempered" .)