It's tough to break it off with sweetheart (6)

I believe the answer is:

severe

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'it's tough' is the definition.
(thesaurus)

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

(Other definitions for severe that I've seen before include "'Strict, harsh (6)'" , "Sharp" , "Harsh and punitive" , "Extreme and difficult to endure" , "Acute" .)

I've seen this clue in the Irish Times.

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