Totally guaranteed to lack a trace of sex appeal (6)
I believe the answer is:
allure
'sex appeal' is the definition.
'totally guaranteed' is the wordplay.
'totally' becomes 'all'.
'guaranteed' becomes 'ure' (I can't justify this - if you can you should give a lot more credence to this answer).
'all'+'ure'='ALLURE'
'to lack a trace of' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for allure that I've seen before include "The power to entice or attract" , "Entice or attract - sounds like a decoy" , "Powerful attraction" , "Charm, magnetism" , "Transport" .)