Call sea a source of power? (4,4)

I believe the answer is:

ring main

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

'a source of power?' is the definition.
I can't judge whether this definition defines the answer.

'call sea' is the wordplay.
'call' becomes 'ring' (telephone call).
'sea' becomes 'main' (synonyms).
'ring'+'main'='RING MAIN'

Can you help me to learn more?

(Other definitions for ring main that I've seen before include "what supplies power" , "Electrical supply" , "Home electricity supply system" , "Electric circuit" , "Syndicate staff installing current" .)

I've seen this clue in The Independent.

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