Fish markets principally in Scottish island, and German one (10)

I believe the answer is:

barramundi

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'fish' is the definition.
(I've seen this before)

'markets principally in scottish island and german one' is the wordplay.
'markets principally' becomes 'm' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'scottish island' becomes 'barra' (I've seen this before).
'and german' becomes 'und' ('and' in German).
'one' becomes 'i' (Roman numeral).
'barra'+'und'='barraund'
'm' inserted inside 'barraund' is 'barramund'.
'barramund'+'i'='BARRAMUNDI'

Can you help me to learn more?

(Another definition for barramundi that I've seen is " Australian fish".)

I've seen this clue in The Times.

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