Slack relatives to help Red Cross? (4,5)

I believe the answer is:

give blood

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'help red cross?' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'slack relatives' is the wordplay.
'slack' becomes 'give' (I've seen this in another clue).
'relatives' becomes 'blood' (I've seen this in another clue).
'give'+'blood'='GIVE BLOOD'

'to' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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