Hard to doctor papers (5)
I believe the answer is:
rigid
'hard' is the definition.
(I've seen this before)
'doctor papers' is the wordplay.
'doctor' becomes 'rig' (both can mean to forge or manipulate).
'papers' becomes 'ID' (identity papers).
'rig'+'id'='RIGID'
'to' is the link.
(Other definitions for rigid that I've seen before include "Frozen" , "starched too much?" , "Uncompromising" , "Stiff and inflexible" , "Stiff, unbending" .)