Hat covering lady's hair essential in power station? (10)

I believe the answer is:

thermopile

'essential in power station?' is the definition.
(device used in a power station)

'hat covering lady's hair' is the wordplay.
'hat' becomes 'tile' (I've seen this before).
'covering' is an insertion indicator.
'lady' becomes 'her' (I've seen this before).
'hair' becomes 'mop' (mop can mean a mass of hair).
'her'+'mop'='hermop'
'tile' enclosing 'hermop' is 'THERMOPILE'.

I've seen this clue in The Times.

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