Deep ditch worker's cutting (9)
I believe the answer is:
trenchant
'cutting' is the definition.
(I've seen this before)
'deep ditch worker's' is the wordplay.
'deep ditch' becomes 'trench' (I've seen this before).
'worker' becomes 'ant' (worker is a type of ant).
'trench'+'ant'='TRENCHANT'
(Other definitions for trenchant that I've seen before include "Keen" , "Clear-cut and hard-hitting" , "Scathing" , "caustic?" , "Fierce" .)