Pigeon in middle of arboretum (5)
I believe the answer is:
homer
'pigeon' is the definition.
(I know that homer is a type of domestic pigeon)
'in middle of arboretum' is the wordplay.
'in' becomes 'home' ('he's in' can mean 'he's home').
'middle of' says to take the centre.
The middle letter of 'arboretum' is 'r'.
'home'+'r'='HOMER'
(Other definitions for homer that I've seen before include "Ancient Greek author" , "Greek poet, c. 800 BC" , "trained to get back to house" , "Great Greek poet who sometimes nods" , "Great Greek author of the Odyssey" .)