Dress up an untidy tunnel (9)
I believe the answer is:
underpass
'tunnel' is the definition.
(underpass is a kind of tunnel)
'dress up an untidy' is the wordplay.
'untidy' is an anagram indicator.
'dress'+'up'+'an'='dressupan'
'dressupan' with letters rearranged gives 'UNDERPASS'.
(Other definitions for underpass that I've seen before include "way across road?" , "Tunnel beneath a road" , "way below the level of others making progress" .)