A number decapitated may be burnt (5)
I believe the answer is:
umber
'burnt' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs can sometimes define each other.
'a number decapitated' is the wordplay.
'decapitated' means to remove the first letter.
'number' with its initial letter taken away is 'UMBER'.
'may be' is the link.
Can you help me to learn more?
(Other definitions for umber that I've seen before include "Earthy colour" , "Dark brown (pigment)" , "Brown earthy substance used as pigment" , "Deep brown" , "Brownish pigment" .)