Skilled aviator goes over river as some land (4)
I believe the answer is:
acre
'some land' is the definition.
(unit of land)
'skilled aviator goes over river' is the wordplay.
'skilled aviator' becomes 'ace' (term for an expert pilot).
'goes over' is an insertion indicator (to go over can mean to wrap around).
'river' becomes 'r'.
'ace' enclosing 'r' is 'ACRE'.
'as' acts as a link.
(Other definitions for acre that I've seen before include "Unit of area - Israeli port" , "Levant sea port" , "Area measure - Israeli port - Brazilian state" , "Old unit of area - old port" , "Ancient city in Israel" .)