Keen to ditch worker (9)
I believe the answer is:
trenchant
'keen' is the definition.
(synonyms)
'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'
'to' acts as a link.
(Other definitions for trenchant that I've seen before include "Fierce" , "Incisive in expression" , "Cutting, forthright" , "Forthright, vigorous" , "Incisive in style" .)