To a very great extent (8)

I think it could be either:

Which length description are you looking for?:

2,3

mightily

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

This is all the clue.

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

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

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