Crossword Genius

To a very great extent (8)

Ross

I believe the answer is:

mightily

'to a very great extent' is the definition.
(I've seen this before)

This is the entire clue.

(Another definition for mightily that I've seen is " To a very great extent".)

I've seen this clue in The Times, The Telegraph, the Irish Times (Simplex) and The Guardian.
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