Upset, French and popular US soldier flare up (6)
I believe the answer is:
ignite
'flare up' is the definition.
(thesaurus)
'upset french and popular us soldier' is the wordplay.
'upset' says the letters should be written backwards.
'french and' becomes 'et' ('and' in the French language).
'popular' becomes 'in'.
'us soldier' becomes 'gi' (I've seen this in other clues).
'et'+'in'+'gi'='etingi'
'etingi' written backwards gives 'IGNITE'.
(Other definitions for ignite that I've seen before include "Cause to catch fire" , "Cause to start burning" , "Swan constellation" , "Put a match to" , "Burn" .)