Finish off ultimate theory (4)

I believe the answer is:

idea

I'm a little stuck... Click here to teach me more about this clue!

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

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

(Other definitions for idea that I've seen before include "a wheeze" , "Something imagined in the mind" , "A suspicion of" , "Purpose" , "Intention" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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