Fail to develop, having a Nationalist supporting Government in the country (8)
I believe the answer is:
stagnate
'fail to develop' is the definition.
(stagnate can mean to fail to develop or go stale)
'a nationalist supporting government in the country' is the wordplay.
'nationalist' becomes 'n' (abbreviation in politics).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'government' becomes 'g' (abbreviation for government).
'in the' indicates putting letters inside.
'country' becomes 'state' (synonyms).
'n' put after 'g' is 'gn'.
'a'+'gn'='agn'
'agn' placed within 'state' is 'STAGNATE'.
'having' is the link.
(Other definitions for stagnate that I've seen before include "Stop shifting" , "Cease to flow or move" , "Become brackish or sluggish" , "Make no move" , "Become foul (water)" .)