A number practise Buddhism (5)
I believe the answer is:
dozen
'a number' is the definition.
(I know that dozen is a type of large integer)
'practise buddhism' is the wordplay.
'practise' becomes 'do' (synonyms).
'buddhism' becomes 'Zen' (Zen Buddhism is an example).
'do'+'zen'='DOZEN'
(Other definitions for dozen that I've seen before include "A number of disciples" , "Quantity of e.g. eggs" , "Set of 12" , "A small number" , "XII" .)