Twelve leaving party for Japanese school (3)
I believe the answer is:
zen
'japanese school' is the definition.
(Japanese school of Buddhism)
'twelve leaving party' is the wordplay.
'twelve' becomes 'dozen' (synonyms).
'leaving' indicates named letters should be taken away.
'party' becomes 'do' (do is a kind of party).
'dozen' with 'do' taken out is 'ZEN'.
'for' is the link.
(Other definitions for zen that I've seen before include "Branch of Buddhism" , "Form of Buddhism that says meditation brings enlightenment" , "School of Buddhism" , "Contemplative school" , "Form of Buddhism stressing meditation" .)