Hard to falsify passport? (5)
I believe the answer is:
rigid
'hard' is the definition.
(thesaurus)
'falsify passport?' is the wordplay.
'falsify' becomes 'rig'.
'passport?' becomes 'id'.
'rig'+'id'='RIGID'
'to' acts as a link.
(Other definitions for rigid that I've seen before include "Stiff, unyielding" , "completely" , "Unable to bend" , "stern" , "Uncompromising" .)