Very old bit in drainage ditch (4)
I believe the answer is:
aged
'very old' is the definition.
(I've seen this before)
'bit in drainage ditch' is the wordplay.
'bit' indicates a hidden word (I've seen 'bit of' mean this).
'AGED' is hidden in the letters of 'in drainage ditch'.
(Other definitions for aged that I've seen before include "Became older" , "Senior" , "Getting on" , "Like Uncle Arly" , "Elderly" .)