Falsehood largely inaccurate, erroneous, first of all (3)
I believe the answer is:
lie
'falsehood' is the definition.
(I know that lie is a type of falsehood)
'largely inaccurate erroneous first of all' is the wordplay.
'first of all' suggests taking the first letters.
The initial letters of 'largely inaccurate erroneous' is 'LIE'.
(Other definitions for lie that I've seen before include "Be prostrate" , "situation" , "Stretch out" , "Tell untruths" , "Falsity" .)