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Energy-filled writer is bit of a tool (4)

Ross

I believe the answer is:

peen

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'bit of a tool' is the definition.
The definition suggests a singular noun which matches the answer.

'energy-filled writer' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'filled' means one lot of letters goes inside another.
'writer' becomes 'pen' (something which writes).
'e' placed within 'pen' is 'PEEN'.

'is' is the link.

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I've seen this clue in The Independent.
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