Leaves before show becomes a ritual (3,8)
I believe the answer is:
tea ceremony
'a ritual' is the definition.
(I've seen this before)
'leaves before show' is the wordplay.
'leaves' becomes 'tea' (as in tea leaves).
'before' says to put letters next to each other.
'show' becomes 'ceremony' (I can't justify this - if you can you should give a lot more credence to this answer).
'tea'+'ceremony'='TEA CEREMONY'
'becomes' is the link.
Can you help me to learn more?
(Other definitions for tea ceremony that I've seen before include "rite?" , "Foreign ritual" .)