One-time cavalry officer carrying old woman’s jewelled headdress (7)
I believe the answer is:
coronet
'woman's jewelled headdress' is the definition.
(type of jewelled headwear)
'one-time cavalry officer carrying old' is the wordplay.
'one-time cavalry officer' becomes 'cornet' (historical rank in a cavalry).
'carrying' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cornet' enclosing 'o' is 'CORONET'.
(Other definitions for coronet that I've seen before include "badge of rank" , "in the band" , "Circular floral head decoration" , "Not genuine" , "Simple crown" .)