Occupying platform, nationalist is to make no progress (8)
I believe the answer is:
stagnate
'to make no progress' is the definition.
(to stagnate is to stop developing or decline)
'occupying platform nationalist' is the wordplay.
'occupying' indicates putting letters inside (inserted letters occupy or enter the word).
'platform' becomes 'stage' (stage is a kind of platform).
'nationalist' becomes 'nat' (short for nationalist).
'stage' going around 'nat' is 'STAGNATE'.
'is' is the link.
(Other definitions for stagnate that I've seen before include "Tan gates could vegetate" , "Become stale from lack of motion" , "Become still and stale, pond or people (8)" , "decline" , "Be idle" .)