Different old woman in set offering a sort of energy (10)

I believe the answer is:

geothermal

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'a sort of energy' is the definition.
I can't tell whether this defines the answer.

'different old woman in set' is the wordplay.
'different' becomes 'other' (similar in meaning).
'old woman' becomes 'ma' (both can mean someone's mother).
'in' means one lot of letters goes inside another.
'set' becomes 'gel' (both can mean to harden).
'other'+'ma'='otherma'
'otherma' inserted into 'gel' is 'GEOTHERMAL'.

'offering' acts as a link.

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I've seen this clue in The Times.

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