Poet's 'void land south of river' (6)
I believe the answer is:
arnold
'poet's' is the definition.
(19th-century poet Matthew Arnold)
'void land south of river' is the wordplay.
'void' says to hollow out the word (remove centre letters) (the word is empty or void of its inside letters).
'south of' says to put letters next to each other.
'river' becomes 'arno' (Italian river).
'land' with its centre removed is 'ld'.
'ld' after 'arno' is 'ARNOLD'.
(Other definitions for arnold that I've seen before include "Essayist" , "Matthew --, English poet" , "-- Wesker, playwright" , "Another poet" , "Sir Malcolm -, composer (Bridge over the River Kwai)" .)