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'm unsure 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 explain 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" .)