Clip loosely inside certain vestment (8)
I believe the answer is:
surplice
'vestment' is the definition.
(I know that surplice is a type of vestment)
'clip loosely inside certain' is the wordplay.
'loosely' is an anagram indicator.
'inside' means one lot of letters goes inside another.
'certain' becomes 'sure' (synonyms).
'clip' is an anagram of 'plic'.
'plic' inserted into 'sure' is 'SURPLICE'.
(Other definitions for surplice that I've seen before include "White vestment worn over a cassock" , "Church vestment" , "Wide-sleeved white garment for clergy" , "Christian vestment" , "Loose white vestment for clergy - sounds excessive?" .)