Cave is reached across river (6)
I believe the answer is:
grotto
'cave' is the definition.
(I've seen this before)
'reached across river' is the wordplay.
'reached' becomes 'gotto' (getting to is a kind of reaching).
'across' means one lot of letters goes inside another.
'river' becomes 'r'.
'gotto' placed around 'r' is 'GROTTO'.
'is' acts as a link.
(Other definitions for grotto that I've seen before include "In company (with); over length of" , "Small cave, perhaps with religious connotations" , "Ornamental cave" , "(Imitation) cave" , "A picturesque cave as in Lourdes (6)" .)