Fish one consumed as main ingredient (5)
I believe the answer is:
basis
'main ingredient' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'fish one consumed' is the wordplay.
'fish' becomes 'bass' (bass is a kind of fish).
'one' becomes 'I' (Roman numeral).
'consumed' is an insertion indicator.
'bass' enclosing 'i' is 'BASIS'.
'as' is the link.
Can you help me to learn more?
(Other definitions for basis that I've seen before include "Ground, foundation" , "Groundwork, foundation" , "Foundation, fundament" , "First principle" , "Form of support" .)