Singers -- choir's foremost lot train badly (9)
I believe the answer is:
contralti
'singers' is the definition.
(contralto is a kind of singer)
'choir's foremost lot train badly' is the wordplay.
'foremost' indicates taking the first letters.
'badly' indicates an anagram (letters in the wrong order).
The first letter of 'choirs' is 'c'.
'lot'+'train'='lottrain'
'lottrain' is an anagram of 'ontralti'.
'c'+'ontralti'='CONTRALTI'
(Other definitions for contralti that I've seen before include "They sing" , "Voices of females" .)