Greek poet in river (5)
I believe the answer is:
homer
'greek poet' is the definition.
(I've seen this before)
'in river' is the wordplay.
'in' becomes 'home' ('he's in' can mean 'he's home').
'river' becomes 'r'.
'home'+'r'='HOMER'
(Other definitions for homer that I've seen before include "Ancient Greek poet" , "Old Greek epic poet" , "Great Greek poet who sometimes nods" , "Racing pigeon?" , "trained to get back to house" .)