Clue: Most of 10 across arranged for group singing "Yakety-Yak" (8)
Answer: COASTERS
Explanation: Definition: group singing "Yakety-Yak"

[*most of*] **TE**(n) **ACROSS **[anag:*arranged*]
 

 
*For the benefit of the younger generation, the Coasters were an American vocal group of the late 50s, who are still around today, although with different group members, and their most well-known hit was "Yakety-Yak"