Charlie, a contract killer missing his last shots, exhausted (7)
I believe the answer is:
jackass
'charlie' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for jackass that I've seen before include "Blockhead" , "One may lark about" , "Charlie" , "Male donkey - fool" , "Stupid person" .)