15 across: The entrance is blackstone, the words begone, elfin, decay, excess carved out of it. (3,3)
The definition part of the clue is ‘The entrance’.
The wordplay part of the clue is that the carved out words are removalisms, they indicate that certain letters need to be removed from a word in order to form the answer proper.
Begone - remove the letter b, b is gone
Elfin - remove the letter l, l is fin and here fin is short for finished
Decay - remove the letter k, the answer must be de-K’d. The prefix ‘de’ is most commonly used this way (Detox, degrease, derail, etc)
Excess - remove the letter s, here it is the ex-s as in the former s but no longer. Like an ex-president
These are all carved out of blackstone and when you remove b, l, k and s from blackstone you are left with actone. The 3,3 is enumeration for the clue and ‘Act one’ is the answer.
The entrance to a play for example would be act one.
It’s a devious clue even by cryptic crossword standards, in this crossword it didn’t even make the top 5 of most difficult clues to solve.