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Less productive element of a battery's substratum (10)

Ross

I believe the answer is:

underlayer

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'a battery's substratum' is the definition.
'underlayer' can be an answer for 'substratum' (thesaurus). I'm unsure of the 'a battery's' bit.

'less productive element' is the wordplay.
'less' becomes 'under' (I've seen this in other clues).
'productive element' becomes 'layer' (I can't explain this - if you can you should give a lot more credence to this answer).
'under'+'layer'='UNDERLAYER'

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.
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