Principal means to make progress (7)
I believe the answer is:
headway
'make progress' is the definition.
(I've seen this before)
'principal means' is the wordplay.
'principal' becomes 'head' (synonyms).
'means' becomes 'way' (a way or means of doing something).
'head'+'way'='HEADWAY'
'to' acts as a link.
(Other definitions for headway that I've seen before include "Progress or success in work" , "make progress" , "Progress or forward motion" , "Advance" , "Ground" .)