Crossword Genius

Parents down, having half usual amount of energy (7)

Ross

I believe the answer is:

fathers

I'm a little stuck... Click here to teach me more about this clue!

'parents' is the definition.
(I know this)

'down having half usual amount of energy' is the wordplay.
I cannot quite understand how this works, but
'amount' could be 'th' (Th is an example) and 'th' is present in the answer.
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is found within the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for fathers that I've seen before include "Takes ref's hat to the parents" , "Sires" , "See 13" .)

I've seen this clue in The Independent.
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