They may safely graze to a well-known tune (5)

I believe the answer is:

sheep

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'tune' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for sheep that I've seen before include "Woolly quadruped" , "Farm animal" , "Woolly animals" , "Woolly animal" , "Members of congregation" .)

I've seen this clue in the Evening Standard.

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