Be surrogate parents for Stephen? (6)

I believe the answer is:

foster

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'be surrogate' is the definition.
(synonyms)

I don't understand how the remainder of the clue works.

(Other definitions for foster that I've seen before include "advance" , "Bring up in forest" , "Member of 7 [ORDER OF MERIT]" , "Nurture and rear" , "Promote, nurture" .)

I've seen this clue in the Evening Standard.

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