Garment not entirely tiptop on chorister (6)
I believe the answer is:
poncho
'garment' is the definition.
(poncho is a kind of garment)
'not entirely tiptop on chorister' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'PONCHO' is hidden within 'tiptop on chorister'.
(Other definitions for poncho that I've seen before include "Mexican cloak" , "South American garment" , "Garment with opening for head" , "Blanket-like garment" , "Type of cloak" .)