Sick? That's vague (3-7)

I believe the answer is:

ill-defined

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

'sick? that's' is the wordplay.
'sick?' becomes 'ill' (similar in meaning).
'that' becomes 'defined' (I can't explain this - if you can you should believe this answer much more).
'ill'+'defined'='ILL-DEFINED'

Can you help me to learn more?

(Another definition for ill-defined that I've seen is " Vague, hazy".)

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