The chief road to progress (7)
I believe the answer is:
headway
'progress' is the definition.
(I know that headway is a type of progress)
'the chief road' is the wordplay.
'the chief' becomes 'head' ('head' can be a synonym of 'chief').
'road' becomes 'way' (I've seen this in other clues).
'head'+'way'='HEADWAY'
'to' acts as a link.
(Other definitions for headway that I've seen before include "make progress" , "Advance" , "Progress or success in work" , "Progress, forward movement" , "Ground" .)