Cave reached when crossing river (6)
I believe the answer is:
grotto
'cave' is the definition.
(I know that grotto is a type of cave)
'reached when crossing river' is the wordplay.
'reached' becomes 'got to' ('get to' can be a synonym of 'reach').
'when crossing' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'gotto' going around 'r' is 'GROTTO'.
(Other definitions for grotto that I've seen before include "Ornamental cave" , "Cavern" , "Attractive natural, or artificial, cave" , "In company (with); over length of" , "Santa's cave" .)