Principal road for progress (7)
I believe the answer is:
headway
'for progress' is the definition.
(headway is a kind of progress)
'principal road' is the wordplay.
'principal' becomes 'head' (synonyms).
'road' becomes 'way' (I've seen this in other clues).
'head'+'way'='HEADWAY'
(Other definitions for headway that I've seen before include "Ground" , "make progress" , "Advance" , "Progress or forward motion" , "Progress, forward movement" .)