Siren in French bank cashier initially stops (7)
I believe the answer is:
enticer
'siren' is the definition.
(thesaurus)
'in french bank cashier initially stops' is the wordplay.
'in french' becomes 'en' ('en' can be French for 'in' e.g. en France).
'bank' becomes 'tier' (I've seen this before**).
'initially' suggests taking the first letters.
'stops' means one lot of letters goes inside another (to stop can mean to block a gap).
The initial letter of 'cashier' is 'c'.
'tier' going around 'c' is 'ticer'.
'en'+'ticer'='ENTICER'
(Other definitions for enticer that I've seen before include "temptress?" , "Who offers temptation" .)