Underlying support (4)
'underlying support' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for basis that I've seen before include "Reason" , "grounds" , "Fundamental part" , "Foundation or support, or a main principle" , "Ground, foundation" .)
(Other definitions for base that I've seen before include "Mean - military installation" , "Mil. operations centre" , "Plinth" , "Mean; HQ" , "Not a noble" .)