Middle of field has thin tree (5)
I believe the answer is:
ebony
'tree' is the definition.
(ebony is a kind of tree)
'middle of field has thin' is the wordplay.
'middle of' means to look at the middle letters.
'has' says to put letters next to each other.
'thin' becomes 'bony' ('bony' can be similar in meaning to 'thin').
The centre of 'field' is 'e'.
'e'+'bony'='EBONY'
(Other definitions for ebony that I've seen before include "Hard dark-coloured wood" , "Tree with blackish wood" , "'Hard, dark wood (5)'" , "Hard block tropical wood" , "Hard wood used for black piano keys and in cabinetwork" .)