Requires river to pass through grottoes (6)
I believe the answer is:
craves
'requires' is the definition.
The answer and definition can be both to do with communicating as well as being verbs in their -s form.
Perhaps you can see a link between them that I don't see?
'river to pass through grottoes' is the wordplay.
I cannot quite see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is present in the answer.
'grottoes' could be 'caves' (I've seen this in another clue) and 'caves' is found in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for craves that I've seen before include "Hungers or longs for" , "Feels a powerful desire" .)