He should start here off Mayo, or else he will go astray, showing fatal weakness (8,4)
I believe the answer is:
achilles heel
'fatal weakness' is the definition.
(I've seen this before)
'he should start here off mayo or else he will go astray' is the wordplay.
I cannot really see how this works, but
'he' could be 'hill' and 'hill' is located in the answer.
This explanation may well be incorrect...
'showing' is the link.
Can you help me to learn more?
(Other definitions for achilles heel that I've seen before include "Personal weak spot" , "Person's weak spot" , "vulnerability" , "Person's vulnerable weakness" , "One's most vulnerable spot" .)