Passage that crosses river ending in Idaho (6)
I believe the answer is:
throat
'passage' is the definition.
(I know that throat is a type of passage)
'that crosses river ending in idaho' is the wordplay.
'crosses' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'ending in' indicates one should take the final letters.
The final letter of 'idaho' is 'o'.
'r'+'o'='ro'
'that' going around 'ro' is 'THROAT'.
(Other definitions for throat that I've seen before include "Narrow entrance" , "Passage to the stomach and lungs" , "Hot rat (anag) - body part" , "It's inside the neck" , "Trachea" .)