Seaweed is containing energy for sheepdogs (7)

I believe the answer is:

kelpies

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

'seaweed is containing energy' is the wordplay.
'seaweed' becomes 'kelp' (I've seen this before).
'containing' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mce).
'is' enclosing 'e' is 'ies'.
'kelp'+'ies'='KELPIES'

'for' acts as a link.

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

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