Rank river in Italy, not Belgium (4)
I believe the answer is:
tier
'rank' is the definition.
(I know that tier is a type of rank)
'river in italy not belgium' is the wordplay.
'river in italy' becomes 'tiber' (river that flows through Rome).
'not' suggests deleting specific letters.
'belgium' becomes 'B' (international vehicle registration code).
'tiber' with 'b' taken away is 'TIER'.
(Other definitions for tier that I've seen before include "One making knot - level" , "One of two or more rows" , "Row, level" , "Row, layer" , "Wedding cake layer" .)