Stiff, cold, decapitated (5)
I believe the answer is:
rigid
'stiff' is the definition.
('rigid' can be a synonym of 'stiff')
'cold decapitated' is the wordplay.
'cold' becomes 'frigid' ('frigid' can be similar in meaning to 'cold').
'decapitated' means to remove the first letter.
'frigid' with its initial letter taken away is 'RIGID'.
(Other definitions for rigid that I've seen before include "Firmly fixed" , "Unable to bend" , "completely" , "Stiff, unmoving" , "starched too much?" .)