Scottish designer's principal clothing label rubbish (10)
I believe the answer is:
mackintosh
'scottish designer's principal' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'clothing label rubbish' is the wordplay.
I cannot really understand how this works, but
'clothing' could be 'mack' (mack is a kind of clothing) and 'mack' is present in the answer.
'rubbish' could be 'tosh' (both can mean nonsense) and 'tosh' is found within the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mackintosh that I've seen before include "Garment" , "waterproof layer" , "Architect" , "providing protection from the elements" , "Weather defence" .)