Holding New Testament, briefly mark a sacred phrase (6)
I believe the answer is:
mantra
'sacred phrase' is the definition.
(sound or phrase used for meditation)
'holding new testament briefly mark a' is the wordplay.
'holding' means one lot of letters goes inside another.
'new testament' becomes 'nt'.
'briefly' means to remove the last letter.
'mark' with its final letter taken away is 'mar'.
'nt' placed within 'mar' is 'mantr'.
'mantr'+'a'='MANTRA'
(Other definitions for mantra that I've seen before include "word to help concentration" , "Repeated incantation" , "Watchword" , "Word or phrase repeated as aid to concentration" , "repeating phrase" .)