Vital suppliers getting socialist family rooms (3,5,5)

I believe the answer is:

red blood cells

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'vital suppliers' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.

'socialist family rooms' is the wordplay.
'socialist' becomes 'red' (red can informally mean a socialist).
'family' becomes 'blood' (blood can mean one's family).
'rooms' becomes 'cells' (cell is a kind of room).
'red'+'blood'+'cells'='RED BLOOD CELLS'

'getting' is the link.

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I've seen this clue in The Daily Express.

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