It's difficult having Scottish island seafood filleted (6)
I believe the answer is:
mulish
'filleted' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs sometimes define each other.
'it's difficult having scottish island seafood' is the wordplay.
I cannot quite see how this works, but
'island' could be 'is' (abbreviation) and 'is' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mulish that I've seen before include "Obstinate like a pack-animal" , "Stubborn" , "unyielding" .)