Letter from a hothead (5)
I believe the answer is:
aitch
'letter' is the definition.
(aitch is a kind of letter of the alphabet)
'a hothead' is the wordplay.
'hothead' becomes 'itch' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'itch'='AITCH'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for aitch that I've seen before include "first bit of hymn?" , "the start of hives?" , "character leading heroes" , "bone" , "Aspirated letter" .)