Quite a tricky puzzle with some novel features. More verifying needed than usual. Will concentrate on what I found the trickier or more unusual clues, but happy to explain any other if asked.
Solving time: 23 mins
* = anagram
ACROSS
1 DA (GO) B A From Buddhism
7 M (ON) AD The number one
10 GREAT MISSENDEN (designers meant)*
13 TEA M I think the clue number is part of the clue here “One may have” in Rugby League, for one. tea = light meal
DOWN
3 BEDMAKER This refers to an apple-pie bed made up as a prank.
14 P (EA GRE) EN Pen = author is v common. eagre = wave is less so. Setter could have used ‘agree’ but this might have been less friendly for the surface reading.
18 S AILING Pre-decimal (1971) currency. Shilling (abbrev s) = 5p in today’s money, colloquially known as a ‘bob’
20 K ENYA The five Ks are a Sikh symbol, Enya a singer
21 H (A) AR Half of Harlem, if I’ve got this right