Congested excavation channel turned over (6)
I believe the answer is:
turgid
'congested' is the definition.
'excavation channel turned over' is the wordplay.
'excavation' becomes 'dig' (synonyms).
'channel' becomes 'rut' (I've seen this before).
'turned over' shows that the letters should be reversed in order.
'dig'+'rut'='digrut'
'digrut' back-to-front is 'TURGID'.
(Other definitions for turgid that I've seen before include "Swollen; pompous (language)" , "Enlarged" , "Of language, pompous and not flowing easily" , "(Of style) bombastic" , "Of language, pompous or inflated" .)