12 down
Leading
composer
slipped
up,
losing
right
to
be
covered
(11)
Leading composer slipped up, losing right to be covered (11)
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hints
The definition is at the end of the clue.
How the clue works
To be covered is your definition, as in padded and fabric-wrapped furniture. You've got UP (leading, as in ahead) followed by HOLST (composer Gustav), then ERRED (slipped up) with the R dropped (losing right), leaving ERED — stick it all together and there you have it.