Eccentric quiet about a feasible formula (8)
I believe the answer is:
equation
'formula' is the definition.
(I've seen this before)
'eccentric quiet about a feasible' is the wordplay.
'eccentric' is an anagram indicator.
'about' indicates putting letters inside.
'feasible' becomes 'on'.
'quiet' anagrammed gives 'equti'.
'equti' enclosing 'a' is 'equati'.
'equati'+'on'='EQUATION'
(Other definitions for equation that I've seen before include "Maths statement with =" , "Ambrose --, author of The Devil's Dictionary" , "Mathemical statement using = sign" , "Mathematical statement that two expressions are equal" , "Statement that two things have the same value" .)