Malice admitted by gymnast in Essex (9)
I believe the answer is:
nastiness
'malice' is the definition.
(nastiness is a kind of malice)
'admitted by gymnast in essex' is the wordplay.
'admitted by' indicates a hidden word.
'NASTINESS' is hidden in the letters of 'gymnast in essex'.
(Other definitions for nastiness that I've seen before include "repugnance" , "Pollution" , "ill humour" , "Malevolence" , "hostility" .)