Singer has one arrangement (5)
I believe the answer is:
basis
'arrangement' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'singer has one' is the wordplay.
'singer' becomes 'bass' (bass is a kind of singer).
'has' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'bass' going around 'i' is 'BASIS'.
Can you help me to learn more?
(Other definitions for basis that I've seen before include "Foundation, fundament" , "Principal element" , "grounds" , "Groundwork, foundation" , "Foundation or support, or a main principle" .)