Going beneath major artery, risk blunder (7)
I believe the answer is:
mistake
'blunder' is the definition.
(thesaurus)
'going beneath major artery risk' is the wordplay.
'going beneath major artery' becomes 'mi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'risk' becomes 'stake' (staking is a kind of risking).
'mi'+'stake'='MISTAKE'
Can you help me to learn more?
(Other definitions for mistake that I've seen before include "Error" , "16 down" , "Oversight" , "Slip" , "Fault" .)