Those with few lines spoiled by a stripe covering fifth of circle (3,7)

I believe the answer is:

bit players

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'those with few lines spoiled by a stripe' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'covering fifth of circle' is the wordplay.
I cannot really understand how this works, but
'covering' could be 'layer' (a layer is covering) and 'layer' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Guardian.

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