Singer has one platform (5)
I believe the answer is:
basis
'platform' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'singer has one' is the wordplay.
'singer' becomes 'bass' (bass is a kind of singer).
'has' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'bass' enclosing 'i' is 'BASIS'.
Can you help me to learn more?
(Other definitions for basis that I've seen before include "Underlying support for a theory" , "First principle" , "Fundamental part" , "Groundwork, foundation" , "Ground, foundation" .)