A short day secures animals (7)

I believe the answer is:

monkeys

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'animals' is the definition.

'a short day secures' is the wordplay.
'a short day' becomes 'mon' (abbreviation for Monday; 'short' suggests an abbreviation).
'secures' becomes 'keys' (I can't explain this - if you can you should believe this answer much more).
'mon'+'keys'='MONKEYS'

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(Other definitions for monkeys that I've seen before include "What you get if you pay 14" , "Long-tailed primates" .)

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

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