Old citizen is a hard worker (6)
I believe the answer is:
trojan
'hard worker' is the definition.
(I've seen this before)
'old citizen is a' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
'a' could be 'an' and 'an' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trojan that I've seen before include "This legendary horse was wooden and hollow" , "To work like one is to work hard" , "This horse once concealed Greek warriors bent on breaking a siege" , "Hard-working, trustworthy person (but, of a horse, untrustworthy!)" , "Hard-working person - coded material inserted to disrupt a computer program" .)