most clues refers to the answer at 1 down, indicating that the
answer is a portmanteau word.
In 21 across and in the surface readings of
some other clues, 1D refers to the boy band One Direction, known as
1D. Fortunately for solvers like myself, whose interest in pop music
begins and ends with a handful of prog rock groups from the 70s,
this second piece of information is not really needed to solve
this excellent puzzle!