Maximally exposed, first covering up hole (8)

I believe the answer is:

bleakest

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'maximally exposed' is the definition.
The answer and definition are not the same part of speech.

'first covering up hole' is the wordplay.
'first' becomes 'best' (similar in meaning).
'covering' is an insertion indicator.
'up hole' becomes 'leak' (I can't justify this - if you can you should believe this answer much more).
'best' going around 'leak' is 'BLEAKEST'.

Can you help me to learn more?

(Other definitions for bleakest that I've seen before include "Most stark" , "Most dismal and desolate" .)

I've seen this clue in The Guardian.

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