Discrimination infiltrating Essex is maddening (6)
I believe the answer is:
sexism
'discrimination' is the definition.
(I know that sexism is a type of discrimination)
'infiltrating essex is maddening' is the wordplay.
'infiltrating' says the answer is hidden in the clue.
'SEXISM' can be found hidden inside 'essex is maddening'.
(Other definitions for sexism that I've seen before include "Discrimination on the basis of gender" , "Discriminatory or abusive behaviour" , "Gender-based discrimination" , "Prejudice on grounds of gender" , "Prejudice based on gender" .)