Principal speaking wouldn't allow hair adornment (8)
I believe the answer is:
headband
'hair adornment' is the definition.
(I've seen this in another clue)
'principal speaking wouldn't' is the wordplay.
'principal' becomes 'head' (synonyms).
'speaking wouldn't' becomes 'band' (I can't explain this - if you can you should believe this answer much more).
'head'+'band'='HEADBAND'
'allow' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for headband that I've seen before include "Something to secure the hair" , "fillet" , "Ribbon worn on top" .)