Austere American soldier getting in free (5)
I believe the answer is:
rigid
'austere' is the definition.
(thesaurus)
'american soldier getting in free' is the wordplay.
'american soldier' becomes 'GI' (informal term for a US soldier).
'getting in' is an insertion indicator.
'free' becomes 'rid' (to be free of something is to be rid of it).
'gi' placed into 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "unwilling to move" , "Hard" , "Unable to bend" , "Frozen" , "stern" .)