Middle of oddly long trench (5)
I believe the answer is:
ditch
'trench' is the definition.
(I know that trench can be written as ditch)
'middle of oddly long' is the wordplay.
'middle of' indicates the central letters.
'long' becomes 'itch' (to long or itch for something is to desire it).
The centre of 'oddly' is 'd'.
'd'+'itch'='DITCH'
(Other definitions for ditch that I've seen before include "Trench, dike" , "Land in water" , "Dug water channel" , "Excavated channel" , "Abandon - trench" .)