Crossword Genius

Unreliable combination of opposites (4,3,5)

Ross

I believe the answer is:

fast and loose

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'unreliable' is the definition.
(thesaurus)

I don't understand the rest of the clue.

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(Other definitions for fast and loose that I've seen before include "Lacking in moral obligation - old cheating game" , "How one trifles with" .)

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