Fail to develop stage, involving nationalist just briefly (8)
I believe the answer is:
stagnate
'fail to develop' is the definition.
(stagnate can mean to fail to develop or go stale)
'stage involving nationalist just briefly' is the wordplay.
'involving' is an insertion indicator.
'nationalist just briefly' becomes 'nat' (short for 'nationalist').
'stage' enclosing 'nat' is 'STAGNATE'.
(Other definitions for stagnate that I've seen before include "Show little activity" , "Cease moving" , "Fester" , "Make no move" , "Grow stale like still water" .)