Trojan more serious about king Greek character imprisons (9)
I believe the answer is:
workhorse
'trojan' is the definition.
(both can mean a hard worker)
'more serious about king greek character imprisons' is the wordplay.
'more serious' becomes 'worse' (in a more serious condition).
'about' is an insertion indicator.
'king' becomes 'K' (chess abbreviation).
'greek character' becomes 'rho' (letter from the Greek alphabet).
'imprisons' is an insertion indicator.
'k' inserted into 'rho' is 'rkho'.
'worse' going around 'rkho' is 'WORKHORSE'.
(Other definitions for workhorse that I've seen before include "tireless toiler" , "Punch maybe" , "Person (or machine) dependably slogging away" , "Reliable toiler" , "giving regular service" .)