Locks up quartet missing feminine charm (7)
I believe the answer is:
enamour
'charm' is the definition.
('enamour' can be a synonym of 'charm')
'locks up quartet missing feminine' is the wordplay.
'locks' becomes 'mane' (I've seen this in other clues**).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'quartet' becomes 'four' ('four' can be a synonym of 'quartet').
'missing' suggests deleting specific letters.
'feminine' becomes 'f' (abbreviation listed in Chambers).
'mane' backwards is 'enam'.
'four' with 'f' removed is 'our'.
'enam'+'our'='ENAMOUR'
(Other definitions for enamour that I've seen before include "Delight" , "Fill with love" , "Charm, captivate" , "Enchant" , "Our name (anag)" .)