Establish the basic cause (4)
I believe the answer is:
root
I believe this is a triple definition!
'establish' is the first definition.
(thesaurus)
'the basic' is the second definition.
(thesaurus)
'cause' is the third definition.
(as in a root cause)
(Other definitions for root that I've seen before include "radical" , "Organ that anchors plants in the ground" , "Origin; carrot perhaps" , "Part of plant drawing nutrients from soil" , "Basic source or origin" .)