Subject to contingency, being informal (4)

I believe the answer is:

iffy

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'subject' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?

I cannot understand how the remainder of the clue works.

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(Other definitions for iffy that I've seen before include "Dubious (colloq.)" , "Doubtful, not definite" , "Questionable - uncertain" , "Dodgy" , "Unsure" .)

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