Bury extreme group making cross (9)
I believe the answer is:
intersect
'cross' is the definition.
(I know that cross can be written as intersect)
'bury extreme group' is the wordplay.
'bury' becomes 'inter' (to inter is to put into the ground).
'extreme group' becomes 'sect' (eg a religious sect).
'inter'+'sect'='INTERSECT'
'making' acts as a link.
(Other definitions for intersect that I've seen before include "Criss-cross" , "Cut across" , "Meet at a point" , "making one cross" , "Divide into parts" .)