Crossing eastern river, passed big snake (7)
I believe the answer is:
serpent
'big snake' is the definition.
(serpent can mean a large snake)
'crossing eastern river passed' is the wordplay.
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'eastern' becomes 'e'.
'river' becomes 'r' (abbreviation for river).
'passed' becomes 'spent' ('spend' can be a synonym of 'pass').
'e'+'r'='er'
'er' put within 'spent' is 'SERPENT'.
(Other definitions for serpent that I've seen before include "Snake - old wind instrument" , "Present for a snake" , "Large snake; sly person" , "Tempting" , "(In literature) a snake" .)