A Parisienne, when meeting unknown quantity, is disturbed (6)
I believe the answer is:
uneasy
'is disturbed' is the definition.
(thesaurus)
'a parisienne when meeting unknown quantity' is the wordplay.
'a parisienne' becomes 'une'.
'when' becomes 'as'.
'meeting' says to put letters next to each other.
'unknown quantity' becomes 'y' (algebra).
'une'+'as'+'y'='UNEASY'
(Other definitions for uneasy that I've seen before include "Anxious, nervous" , "Uncomfortable, apprehensive" , "Nervous, worried" , "Anxious, unquiet" , "Restless, troubled" .)