Essential to slip on choir cloak (6)
I believe the answer is:
poncho
'cloak' is the definition.
(poncho is a kind of cloak)
'essential to slip on choir' is the wordplay.
'essential to' indicates the answer is hidden within the clue (letters in the inside or essence of the clue).
'PONCHO' can be found hidden inside 'slip on choir'.
(Other definitions for poncho that I've seen before include "South American cloak with slit for the head" , "Chihuahua fashion?" , "Blanket-like cloth with centre slit, worn as cloak" , "Mexican cloak" , "Blanket-like cloak with slit for head" .)