Main line terminals (8)

I believe the answer is:

seaports

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'terminals' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.

'main line' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'line' becomes 'ports' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sea'+'ports'='SEAPORTS'

Can you help me to learn more?

I've seen this clue in The Globe and Mail.

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