Prosecute union leader in Home Counties (3)
I believe the answer is:
sue
'prosecute' is the definition.
(thesaurus)
'union leader in home counties' is the wordplay.
'leader' says to take the initial letters.
'in' indicates putting letters inside.
'home counties' becomes 'se'.
The first letter of 'union' is 'u'.
'u' put inside 'se' is 'SUE'.
(Other definitions for sue that I've seen before include "Charge" , "Little Susan" , "Take a casa against (3)" , "Bring charges against - girl" , "Prosecute; girl's name" .)