Ditch worker for being acerbic (9)
I believe the answer is:
trenchant
'being acerbic' is the definition.
(I've seen this before)
'ditch worker' is the wordplay.
'ditch' becomes 'trench' (synonyms).
'worker' becomes 'ant' (worker is a type of ant).
'trench'+'ant'='TRENCHANT'
'for' is the link.
(Other definitions for trenchant that I've seen before include "Fierce" , "Acerbic" , "Cutting, forthright" , "Clear" , "blunt" .)