Falsify documentation for firm (5)
I believe the answer is:
rigid
'for firm' is the definition.
(I've seen this in another clue)
'falsify documentation' is the wordplay.
'falsify' becomes 'rig'.
'documentation' becomes 'ID' (identification documents).
'rig'+'id'='RIGID'
(Other definitions for rigid that I've seen before include "Stiff, unyielding" , "Frozen" , "Stiff, unmoving" , "Hard" , "not relaxing" .)