Illustrated River Dee, clear down the middle (9)
I believe the answer is:
explained
'illustrated' is the definition.
(illustrate can mean to explain or show)
'river dee clear down the middle' is the wordplay.
'river' becomes 'exe' (River Exe).
'dee' becomes 'd'.
'clear' becomes 'plain' (similar in meaning).
'down the middle' indicates putting letters inside.
'exe'+'d'='exed'
'exed' enclosing 'plain' is 'EXPLAINED'.
(Other definitions for explained that I've seen before include "Made clear, gave reasons for" , "Made comprehensible" , "opened up" , "Elucidated, clarified" .)