Cutting ditch by a worker (9)
I believe the answer is:
trenchant
'cutting' is the definition.
(I've seen this before)
'ditch by a worker' is the wordplay.
'ditch' becomes 'trench' (synonyms).
'by' means one lot of letters go next to another.
'a worker' becomes 'ant' (worker is a type of ant).
'trench'+'ant'='TRENCHANT'
(Other definitions for trenchant that I've seen before include "Incisive in style" , "Cutting, forthright" , "Acerbic" , "blunt" , "Vigorous and effective" .)