Hard water? That depends (6)

I believe the answer is:

icicle

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'hard water? that depends' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps there's a link between them I don't understand?

This is the entire clue.

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(Other definitions for icicle that I've seen before include "Winter spike" , "Tree D [DECORATION]" , "A pendant spike of frozen water" , "Hanging drop of cold water" , "Spear-like shape formed by the freezing of dripping water" .)

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

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