Length of hair, mine, captivating the French (5)
I believe the answer is:
plait
'length of hair' is the definition.
'mine captivating the french' is the wordplay.
'mine' becomes 'pit' (down the pit).
'captivating' is an insertion indicator (I've seen this in other clues).
'the french' becomes 'la'.
'pit' placed around 'la' is 'PLAIT'.
(Other definitions for plait that I've seen before include "Hairdo formed by braiding" , "A twine of hair" , "Braided pigtail" , "Intertwine" , "Interwoven length of hair" .)