Hard worker in the city sacked long ago (6)
I believe the answer is:
trojan
'hard worker' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for trojan that I've seen before include "Computer threat" , "This legendary horse was wooden and hollow" , "This Horse was a hollow invader" , "This horse once concealed Greek warriors bent on breaking a siege" , "Native of Troy" .)