Ditch worker for cutting (9)
I believe the answer is:
trenchant
'cutting' is the definition.
(I've seen this before)
'ditch worker' is the wordplay.
'ditch' becomes 'trench' (trench is a kind of ditch).
'worker' becomes 'ant' (worker is a type of ant).
'trench'+'ant'='TRENCHANT'
'for' acts as a link.
(Other definitions for trenchant that I've seen before include "Acerbic" , "Distinctly defined" , "Keen" , "Clear-cut and hard-hitting" , "Distinct" .)