Lettuce having hidden depths? (7)

I believe the answer is:

iceberg

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'lettuce' is the definition.
(I know that iceberg is a type of lettuce)

I can't explain the remainder of the clue.

(Other definitions for iceberg that I've seen before include "Large mass of floating ice" , "Mass carved from a glacier" , "Variety of lettuce" , "Floating shipping hazard" , "Mass calved from a glacier" .)

I've seen this clue in The Sun.

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