Hairdo Doctor Faustus has pinned back (4)
I believe the answer is:
afro
'hairdo' is the definition.
(afro is a kind of hairdo)
'doctor faustus has pinned back' is the wordplay.
I cannot really see how this works, but
'back' could be 'fro' (synonyms) and 'fro' is found within the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for afro that I've seen before include "Hair cut" , "Type of 18" , "Big hairstyle" , "Style of haircut" , "Roundish hairstyle" .)