They keep an eye on the offspring of mature animals (6)

I believe the answer is:

adults

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'mature animals' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for adults that I've seen before include "A-rated film audience" , "Fully grown animals" , "Mature creatures" , "Mature persons" , "Grown-ups, mature people" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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