A number practise a form of Buddhism (5)
I believe the answer is:
dozen
'a number' is the definition.
(dozen is a kind of number)
'practise a form of buddhism' is the wordplay.
'practise' becomes 'do' ('do' can be a synonym of 'practise').
'a form of buddhism' becomes 'zen' (I've seen this before).
'do'+'zen'='DOZEN'
(Other definitions for dozen that I've seen before include "See 4" , "Set of twelve" , "Set of 12" , "Group of twelve" , "Twelve (or thirteen for a baker?)" .)