Very likely robust (4)

I believe the answer is:

well

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'very' is the definition.
(can informally mean 'very')

I don't understand the remainder of the clue.

(Other definitions for well that I've seen before include "Healthy - source of water" , "abundantly" , "Bore; very much" , "We shall" , "How one might like one's steak?" .)

I've seen this clue in The Sun.

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