National application, then change in command (6)
I believe the answer is:
ethnic
'national' is the definition.
(thesaurus)
'then change in command' is the wordplay.
'change' indicates anagramming the letters.
'in command' becomes 'ic' (military abbreviation).
'then' anagrammed gives 'ethn'.
'ethn'+'ic'='ETHNIC'
'application' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ethnic that I've seen before include "Relating to national and cultural origins" , "Relating to a cultural grouping" , "Relating to cultural groups" , "Of nations and races" , "Characteristic of a human group" .)