Underlying support (5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underlying support</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)