Vestment in large container left in reserve (7)
I believe the answer is:
tunicle
'vestment' is the definition.
(I've seen this before)
'large container left in reserve' is the wordplay.
'large container' becomes 'tun' (I've seen this in another clue).
'left' becomes 'L' (common abbreviation).
'in' means one lot of letters goes inside another.
'reserve' becomes 'ice' (both can mean aloofness).
'l' inserted within 'ice' is 'icle'.
'tun'+'icle'='TUNICLE'
'in' acts as a link.
(Other definitions for tunicle that I've seen before include "... suitable apparel [18D>BISHOP]" , "Liturgical vestment" .)