It's said principal wouldn't allow adornment in hair (8)
I believe the answer is:
headband
'hair' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it's said principal wouldn't allow adornment' is the wordplay.
I cannot really see how this works, but
'principal' could be 'head' (synonyms) and 'head' is found within the answer.
'adornment' could be 'band' (band is a kind of adornment) and 'band' is found within the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for headband that I've seen before include "Ribbon worn on top" , "fillet" , "Something to secure the hair" .)