Additionally, where one may apply a shoehorn (2,4)

I believe the answer is:

to boot

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'additionally' is the definition.
(synonyms)

I can't explain the remainder of the clue.

(Other definitions for to boot that I've seen before include "Moreover, in addition to (2,4)" , "In addition - using heavy footwear" , "furthermore" , "As well" , "Besides, in addition - think footwear" .)

I've seen this clue in The Guardian.

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