Con artist's first sting a blunder (7)
I believe the answer is:
against
'con' is the definition.
(thesaurus)
'artist's first sting a blunder' is the wordplay.
'first' says to take the initial letters.
'blunder' indicates anagramming the letters.
The initial letter of 'artists' is 'a'.
'sting'+'a'='stinga'
'stinga' with letters rearranged gives 'gainst'.
'a'+'gainst'='AGAINST'
(Other definitions for against that I've seen before include "Not supporting" , "Placed in direct contact with" , "In competition with" , "Facing" , "Opposed" .)