River leaves long grotto (4)
I believe the answer is:
cave
'grotto' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for cave that I've seen before include "Troglodyte's home" , "Hollow - warning" , "Natural subterranean opening" , "Subject of speleology" , "Warning - grotto" .)