Flatten headless, evil-tempered dwarf (4)
I believe the answer is:
roll
'flatten' is the definition.
(rolling is a kind of flattening)
'headless evil-tempered dwarf' is the wordplay.
'headless' means to remove the first letter.
'evil-tempered dwarf' becomes 'troll'.
'troll' with its first letter taken away is 'ROLL'.
(Other definitions for roll that I've seen before include "Move like a ball" , "Revolve - piece of bread" , "Official list; cylinder" , "cylindrical moulding" , "Bread - list - drumbeats" .)