A Parisian's very slowly opening a lock (5)
I believe the answer is:
tress
'a lock' is the definition.
(both can mean a piece of hair)
'a parisian's very slowly opening' is the wordplay.
'a parisian's very' becomes 'tres' (I've seen this in another clue).
'opening' suggests taking the first letters.
The first letter of 'slowly' is 's'.
'tres'+'s'='TRESS'
(Other definitions for tress that I've seen before include "Braid of hair" , "Long lock or plait of hair" , "Obsolete support" , "Bit of hair" , "Curl - lock" .)