Yemeni's neighbour soon to be rising (5)
I believe the answer is:
omani
'yemeni's neighbour' is the definition.
'soon to be rising' is the wordplay.
'soon' becomes 'in a mo' (in a mo).
'to be rising' shows that the letters should be reversed in order (going up in a down clue).
'inamo' reversed gives 'OMANI'.
(Other definitions for omani that I've seen before include "Middle East national" , "Asian" , "Citizen of a country, capital Muscat" , "one from the Middle East" , "One from Muscat" .)