Crossing river, reached cave (6)
I believe the answer is:
grotto
'cave' is the definition.
(I know that cave can be written as grotto)
'crossing river reached' is the wordplay.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation).
'reached' becomes 'got to' ('get to' can be a synonym of 'reach').
'r' inserted inside 'gotto' is 'GROTTO'.
(Other definitions for grotto that I've seen before include "Ornamental cave" , "fancy underground area" , "A picturesque cave as in Lourdes (6)" , "Cavern" , "Small cave, perhaps with religious connotations" .)