The fundamental ingredient of cobbler essentially unaltered (5)
I believe the answer is:
basis
'the fundamental ingredient' is the definition.
'cobbler essentially unaltered' is the wordplay.
'essentially' indicates the central letters.
'unaltered' becomes 'asis' (ie something left 'as is').
The middle letter of 'cobbler' is 'b'.
'b'+'asis'='BASIS'
'of' acts as a link.
(Other definitions for basis that I've seen before include "Fundamental part" , "Form of support" , "Reason" , "Underlying support for a theory" , "First principle" .)