A small number study a school of Buddhism (5)
I believe the answer is:
dozen
'a small number' is the definition.
(I've seen this before)
'study a school of buddhism' is the wordplay.
'study' becomes 'do' (to 'do' a subject at university is to study it).
'a school of buddhism' becomes 'zen' (I've seen this in another clue).
'do'+'zen'='DOZEN'
(Other definitions for dozen that I've seen before include "Quantity of e.g. eggs" , "A number of disciples" , "Set of 12" , "Set of twelve" , "Twelve (or thirteen for a baker?)" .)