But this Scottish island is not in the Tweed! (6)
I believe the answer is:
harris
This could be a double definition.
'but this scottish island is not' is the first definition.
'harris' can be an answer for 'scottish island' (I've seen this before). I am not sure about the rest of the definition.
'tweed' is the second definition.
(Harris tweed is an example)
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for harris that I've seen before include "somewhere in the Hebrides" , "Joel Chandler --, creator of Brer Rabbit" , "Hebridean area associated with tweed" , "Isle known for tweed" , "Hebridean isle associated with tweed" .)