Falsify papers for firm (5)
I believe the answer is:
rigid
'firm' is the definition.
(I've seen this in another clue)
'falsify papers' is the wordplay.
'falsify' becomes 'rig'.
'papers' becomes 'ID' (identification papers).
'rig'+'id'='RIGID'
'for' is the link.
(Other definitions for rigid that I've seen before include "unwilling to move" , "Stiff, unyielding" , "completely" , "Incapable of or resistant to bending" , "Frozen" .)