Crossword Genius

Like a minority group’s undertaking blocking river (9)

Ross

I believe the answer is:

factional

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'like a minority group's' is the definition.
I can't judge whether this definition defines the answer.

'undertaking blocking river' is the wordplay.
'undertaking' becomes 'action'.
'blocking' means one lot of letters goes inside another (block can mean to plug a gap in something).
'river' becomes 'fal' (Cornish river).
'action' inserted into 'fal' is 'FACTIONAL'.

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I've seen this clue in The Times.
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