Hard to falsify papers (5)
I believe the answer is:
rigid
'hard' is the definition.
(I've seen this before)
'falsify papers' is the wordplay.
'falsify' becomes 'rig'.
'papers' becomes 'id' (identification papers).
'rig'+'id'='RIGID'
'to' acts as a link.
(Other definitions for rigid that I've seen before include "Stiff and inflexible" , "Firmly fixed" , "starched too much?" , "Stiff, unbending" , "Not flexible" .)