Lifeless King's body finally left by the Queen (5)
I believe the answer is:
Here is my best explanation:
'lifeless' is the definition.
'king's body finally left by the queen' is the wordplay.
'king's body' becomes 'in' (I am not sure about this - if you are sure you should believe this answer much more).
'finally' indicates one should take the final letters.
'by' says to put letters next to each other.
'the queen' becomes 'er' (Elizabeth Regina).
The final letter of 'left' is 't'.
't' put after 'er' is 'ert'.
(Other definitions for inert that I've seen before include "(Chem.) not reactive", "Unresponsive", "Not chemically active", "Inanimate", "dead", "Devoid of motion or change", "Like some gases", "Lifeless or static", "Dormant, lifeless" and "Without the strength to move".)