Singer tours one ground (5)
I believe the answer is:
basis
'ground' is the definition.
('basis' can be a synonym of 'ground')
'singer tours one' is the wordplay.
'singer' becomes 'bass' (bass is a kind of singer**).
'tours' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'bass' placed around 'i' is 'BASIS'.
(Other definitions for basis that I've seen before include "Principle on which a theory is founded" , "Foundation of a theory or process" , "Foundation, fundament" , "Groundwork, foundation" , "Form of support" .)