Crossword Genius

Special organisation to follow small rainbow (6)

Ross

I believe the answer is:

sundog

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'small rainbow' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'special organisation to follow' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'organisation' becomes 'UN' (the United Nations is an organisation).
'to follow' becomes 'dog' (dog can mean to follow or torment).
's'+'un'+'dog'='SUNDOG'

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I've seen this clue in The Irish Times.
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