Mark the end of Athens, perhaps, carrying cross (6)
I believe the answer is:
stigma
'mark the end' is the definition.
'stigma' can be an answer for 'mark' (I've seen this before). I'm not sure about the 'the end' bit.
'athens perhaps carrying cross' is the wordplay.
'athens perhaps' becomes 'sigma' (I can't explain this - if you can you should give a lot more credence to this answer).
'carrying' is an insertion indicator.
'cross' becomes 't' (shape).
'sigma' enclosing 't' is 'STIGMA'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stigma that I've seen before include "part of plant" , "Disgraceful mark" , "Mark of infamy or disgrace" , "Sign of disgrace" , "Symbol of disgrace or infamy" .)