American soldier coming in to deliver for firm (5)
I believe the answer is:
rigid
'for firm' is the definition.
(I've seen this in another clue)
'american soldier coming in to deliver' is the wordplay.
'american soldier' becomes 'GI' (informal term for a US soldier).
'coming in to' indicates putting letters inside.
'deliver' becomes 'rid' (to rid from is to free or deliver from).
'gi' placed inside 'rid' is 'RIGID'.
(Other definitions for rigid that I've seen before include "Uncompromising" , "Stiff and inflexible" , "Frozen" , "Stiff, unyielding" , "Hard" .)