In the main a submarine hazard? (7)

I believe the answer is:

iceberg

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'hazard?' is the definition.
(I have seen 'Main hazard' mean 'iceberg' so perhaps 'hazard' could also mean 'iceberg')

I don't understand how the rest of the clue works.

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(Other definitions for iceberg that I've seen before include "sort of lettuce" , "One destroyed the Titanic" , "Large floating mass of frozen water" , "threat to ship" , "Calf's parent" .)

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

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