Dancers lie at first in bed, pectorals rippling (5,2,6)
I believe the answer is:
corps de ballet
'dancers' is the definition.
(I've seen this before)
'lie at first in bed pectorals rippling' is the wordplay.
'at first' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'rippling' is an anagram indicator.
The first letter of 'lie' is 'l'.
'bed'+'pectorals'='bedpectorals'
'bedpectorals' anagrammed gives 'corpsdebalet'.
'l' going into 'corpsdebalet' is 'CORPS DE BALLET'.
(Other definitions for corps de ballet that I've seen before include "dancers" , "dance troupe" , "Dancing troupe" .)