False witness to a rail crash (4)
I believe the answer is:
liar
'false witness' is the definition.
(thesaurus)
'a rail crash' is the wordplay.
'crash' indicates an anagram.
'rail' anagrammed gives 'LIAR'.
'to' acts as a link.
(Other definitions for liar that I've seen before include "Not a truth-sayer!" , "not straight [one]" , "One telling untruths" , "Teller of untruths" , "Dishonest person" .)