Transvestite clothing is an obstacle to progress (4)
I believe the answer is:
drag
I believe this is a double definition.
'transvestite clothing' is the first definition.
'an obstacle to progress' is the second definition.
(I've seen this before)
'is' acts as a link.
(Other definitions for drag that I've seen before include "Hunt without foxes" , "Transvestite's garb" , "such a nuisance" , "Short smoke" , "car" .)