Illustration on page 50: a hollow tree (5)
I believe the answer is:
plate
'illustration' is the definition.
(I know that plate is a type of illustration)
'page 50 a hollow tree' is the wordplay.
'page' becomes 'p'.
'50' becomes 'l' (Roman numeral).
'hollow' says to hollow out the word (remove centre letters) (the word is hollowed out).
'tree' with its centre removed is 'te'.
'p'+'l'+'a'+'te'='PLATE'
'on' is the link.
(Other definitions for plate that I've seen before include "impression" , "From which food is eaten or served" , "Coat with metal" , "This glass is thick as for shop window" , "South American river" .)