Some relief is in the foundation (5)
I believe the answer is:
basis
'foundation' is the definition.
(I know that foundation can be written as basis)
'some relief is' is the wordplay.
'some relief' becomes 'bas' (bas relief is a kind of relief).
'bas'+'is'='BASIS'
'in the' acts as a link.
(Other definitions for basis that I've seen before include "Principle on which a theory is founded" , "grounds" , "Groundwork, foundation" , "First principle" , "Foundation or support, or a main principle" .)