We hear ocean adjoined river of old (6)
I believe the answer is:
senile
'old' is the definition.
(similar in meaning)
'we hear ocean adjoined river' is the wordplay.
I cannot quite see how this works, but
'river' could be 'nile' (Nile River is an example) and 'nile' is found in the answer.
The remaining letters 'se' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for senile that I've seen before include "Doddering" , "Old and infirm" , "Mentally weak due to old age" , "In one's dotage" , "Old and doddery" .)