Her mascot displayed front cover once (9)
I believe the answer is:
stomacher
'front cover once' is the definition.
(piece of cloth once worn over a person's front)
'her mascot displayed' is the wordplay.
'displayed' indicates anagramming the letters (letters displayed in another way).
'her'+'mascot'='hermascot'
'hermascot' with letters rearranged gives 'STOMACHER'.
(Other definitions for stomacher that I've seen before include "Decorative panel on front of gown or bodice" , "clothing" , "ornament worn under bodice" .)