Graduate leaves large church compound (6)
I believe the answer is:
silica
'compound' is the definition.
(I've seen this before)
'graduate leaves large church' is the wordplay.
'graduate' becomes 'ba'.
'leaves' suggests deleting specific letters.
'large church' becomes 'basilica'.
'basilica' with 'ba' taken away is 'SILICA'.
(Other definitions for silica that I've seen before include "Compound in quartz" , "Oxide occurring as quartz and used in making glass" , "Hard colourless compound from rocks" , "Eg, quartz" , "Common base of rock" .)