Rural river enters Niagara explosively (8)
I believe the answer is:
agrarian
'rural' is the definition.
(similar in meaning)
'river enters niagara explosively' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'enters' is an insertion indicator.
'explosively' is an anagram indicator.
'niagara' is an anagram of 'agarian'.
'r' going within 'agarian' is 'AGRARIAN'.
(Other definitions for agrarian that I've seen before include "Relating to agriculture" , "Of agricultural land or its cultivation" , "Relating to the cultivation of land" , "in the country" , "Relating to cultivated land" .)