Big pile of Oxford University books collected by principal (8)
I believe the answer is:
mountain
'big pile' is the definition.
(mountain can mean a large amount of something)
'oxford university books collected by principal' is the wordplay.
'oxford university' becomes 'OU'.
'books' becomes 'NT' (New Testament).
'collected by' indicates putting letters inside.
'principal' becomes 'main' (both can mean major).
'ou'+'nt'='ount'
'ount' inserted within 'main' is 'MOUNTAIN'.
'of' is the link.
(Other definitions for mountain that I've seen before include "eg Everest" , "with pile of 1d [ADVENTURE]" , "Natural elevation of the earth's surface" , "High peak" , "eg Ben Nevis" .)