Crossword Genius

Unimportant, short-lived ménage à trois? (8)

Ross

I believe the answer is:

trifling

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'unimportant' is the definition.
(I've seen this before)

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

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(Other definitions for trifling that I've seen before include "Dallying; petty" , "Unimportant, trivial" , "Frivolous" , "insignificant" , "Fooling around; unimportant" .)

I've seen this clue in The Times.
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