There's possibly nigh on a hundred worn in the hair (7)
I believe the answer is:
chignon
'hair' is the definition.
(chignon is a kind of hair)
'there's possibly nigh on a hundred worn' is the wordplay.
'there's' means one lot of letters goes inside another (I've seen this in other clues).
'possibly' is an anagram indicator.
'on' means one lot of letters go next to another.
'a hundred' becomes 'C' (Roman numerals).
'worn' becomes 'on'.
'nigh' is an anagram of 'hign'.
'c' put next to 'on' is 'con'.
'hign' inserted into 'con' is 'CHIGNON'.
'in the' is the link.
(Other definitions for chignon that I've seen before include "Arrangement of coil of hair" , "Roll of hair worn at the nape" , "Coil of hair at the back of the head" , "Roll of hair worn at the back of the head" , "Topknot" .)