Ruler in mess, enemies finally advancing (4)
I believe the answer is:
shah
'ruler in mess' is the definition.
'shah' can be an answer for 'ruler' (shah is a kind of ruler). I am not certain of the 'in mess' bit.
'enemies finally advancing' is the wordplay.
'finally' indicates one should take the final letters.
'advancing' becomes 'hah' (I can't justify this - if you can you should give a lot more credence to this answer).
The last letter of 'enemies' is 's'.
's'+'hah'='SHAH'
Can you help me to learn more?
(Other definitions for shah that I've seen before include "King of Persia" , "Title of Iranian monarch" , "Persian monarch" , "Old king" , "Former ruler of Persia" .)