Puzzletome Crossword Puzzles

April 2018 Monthly Crossword
April

Six grid entries contain the same element (not consecutive, but in order) which is missing from the wordplay. All of the other clued entries contain a superfluous word, the initials of which (in clue order) spell out a phrase. This should lead to the deduction of the two unclued entries (author and protagonist).
Grid
Across
1 Black Rocks supplementary note for artillery guns? (6)
4 To be deduced (6)
8 Carbon (unknown volume) works best with a single observer? (7)
9 Silent under a hospital radio transmission (7)
11 Housemaid's dreadful Aug 1 run (2,4,4)
12 Hunger for leader leaving Internazionale ground (4)
13 Mites growing in sea (Caribbean) (5)
14 Runner Sebastian to jump (8)
16 Hazardous instrument to exercise copper in the ghetto (8)
18 Block and tackle's initially absurd role in search (5)
20 Smart clothes hang elegantly at first (4)
21 End of nasty person's characteristics (3,7)
23 Often American friend snatches up opportunity (7)
24 To be deduced (7)
25 Lazed in lots of love, luxury and light (6)
26 Poland border to demand guarantee (6)
Down
1 Marshy stream draining almost complete after inlet (5)
2 Airstrip One end returns (after 101) to basic life? (7)
3 She delivers pin, right? (9)
5 Solid container for name of Yamaha organ (5)
6 Anger rising in time off for indolent retirees? (7)
7 In glimpses of Loch Ness yearly return I yell / shout with emotion (9)
10 Pie filling meant African mice infestation (9)
13 Judgement of father and son initially atrocious (9)
15 Bits of pulp enable manufacturing literature? (5,4)
17 Table white to prompt pole dance (3,4)
19 Official exploited, runner withdrew (7)
21 Dig edges of dike blocking one incubus from wicked return (5)
22 Free licence for facilities to start/end (5)