Devilish letter from a friend (5)
I believe the answer is:
fiend
'a friend' is the definition.
(fiend is a kind of friend)
'devilish letter' is the wordplay.
I cannot quite understand how this works, but
'letter' could be 'en' (I've seen this in other clues) and 'en' is found in the answer.
The remaining letters 'fid' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'from' acts as a link.
Can you help me to learn more?
(Other definitions for fiend that I've seen before include "Devilish person" , "Evil supernatural being" , "Evil spirit, demon" , "Operatic super-soprano" , "Wicked and inhuman person" .)