Complicated love affair that may offer a warning (8)

I believe the answer is:

triangle

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'love affair that may offer a warning' is the definition.
'triangle' can be an answer for 'love affair' (thesaurus). I am unsure of the remainder of the definition.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for triangle that I've seen before include "Constellation - three-sided figure" , "Integral (anag) - figure" , "Integral (anag) - geometrical shape" , "kitchen item" , "Figure - percussion instrument" .)

I've seen this clue in The Times.

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