Lady briefly taking you for a Parisian in fact (5)
I believe the answer is:
datum
'fact' is the definition.
(thesaurus)
'lady briefly taking you for a parisian' is the wordplay.
'lady' becomes 'dame' ('dame' can be a synonym of 'lady').
'briefly' means to remove the last letter.
'taking' is an insertion indicator.
'you for a parisian' becomes 'tu' ('you' in French).
'dame' with its last letter removed is 'dam'.
'dam' placed around 'tu' is 'DATUM'.
'in' acts as a link.
(Other definitions for datum that I've seen before include "Premiss, thing taken for granted" , "Basic fact" , "Given item" , "Minimal information" , "Fixed starting point for measurements" .)