Cavern reached when crossing river (6)
I believe the answer is:
grotto
'cavern' is the definition.
(I know that cavern can be written as grotto)
'reached when crossing river' is the wordplay.
'reached' becomes 'got to' (getting to is a kind of reaching).
'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 "It's left on Dec 24 by Santa and his reindeer" , "Cavern" , "Type of cave, often with religious significance" , "Ornamental cave" , "Artificially or naturally ornamented cave" .)