Distrust that Parisian railway (5)
I believe the answer is:
query
'distrust' is the definition.
(thesaurus)
'that parisian railway' is the wordplay.
'that parisian' becomes 'que' ('that' in French).
'railway' becomes 'ry' (abbreviation for railway).
'que'+'ry'='QUERY'
(Other definitions for query that I've seen before include "Doubt" , "Question mark (printing)" , "(Pose a) question" , "Inquiry" .)