Thirteen cooks do a form of Buddhism (6,5)
I believe the answer is:
bakers dozen
'thirteen' is the definition.
(synonyms)
'cooks do a form of buddhism' is the wordplay.
'cooks' becomes 'bakers' (I can't explain this - if you can you should believe this answer much more).
'a form of buddhism' becomes 'zen' (I've seen this before).
'bakers'+'do'+'zen'='BAKERS DOZEN'
Can you help me to learn more?
(Other definitions for bakers dozen that I've seen before include "It might be a bloomer" , "Unlucky number of loaves?" , "Additional fare here" , "Thirteen loaves?" , "13" .)