Ceremonial clothing stocks not popular? (9)
I believe the answer is:
vestments
'ceremonial clothing' is the definition.
(ceremonial clothing worn by a priest)
'stocks not popular?' is the wordplay.
'stocks' becomes 'investments' (as in stocks and shares).
'not' is a deletion indicator.
'popular?' becomes 'in' ('in' can mean trendy).
'investments' with 'in' removed is 'VESTMENTS'.
(Other definitions for vestments that I've seen before include "Clerical clothes" , "Religious garments" , "Ceremonial robes as worn by priests" .)