Stubborn American soldier in Republican papers (5)
I believe the answer is:
rigid
'stubborn' is the definition.
(thesaurus)
'american soldier in republican papers' is the wordplay.
'american soldier' becomes 'GI' (informal term for a US soldier).
'in' indicates putting letters inside.
'republican' becomes 'R' (abbreviation).
'papers' becomes 'id' (short for identification papers).
'r'+'id'='rid'
'gi' inserted within 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "Stiff and inflexible" , "stern" , "not relaxing" , "Frozen" , "Incapable of or resistant to bending" .)