Money withdrawn invested in flat power unit (8)

I believe the answer is:

megawatt

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'unit' is the definition.
(megawatt is a kind of unit)

'money withdrawn invested in flat power' is the wordplay.
'money' becomes 'm' (abbreviation in economics).
'withdrawn' shows that the letters should be reversed in order.
'invested in flat' becomes 'age' (I can't justify this - if you can you should give a lot more credence to this answer).
'power' becomes 'watt'.
'age' back-to-front is 'ega'.
'm'+'ega'+'watt'='MEGAWATT'

Can you help me to learn more?

(Other definitions for megawatt that I've seen before include "Unit of electrical power" , "measure of power" .)

I've seen this clue in The Guardian.

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