Blue part of the Emerald Isle (4)

I believe the answer is:

down

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'blue' is the definition.
(both can mean sad)

'part of the emerald isle' is the wordplay.
'part' becomes 'do' (do is a kind of part).
'of the emerald isle' becomes 'wn' (I can't explain this - if you can you should believe this answer much more).
'do'+'wn'='DOWN'

Can you help me to learn more?

(Other definitions for down that I've seen before include "See 18" , "Soft feathers (like this clue)" , "Depressed - feathers" , "See 1" , "Melancholy" .)

I've seen this clue in the Evening Standard.

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