Reaper uses it but not well, left half behind! (6)
I believe the answer is:
sickle
'reaper' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'it but not well left half behind' is the wordplay.
I cannot quite see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'not well' could be 'sick' ('sick' is the opposite to 'well') and 'sick' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'uses' acts as a link.
Can you help me to learn more?
(Other definitions for sickle that I've seen before include "One stands for Communist" , "Tool for cutting grass or crops" , "Crescent-shaped hand tool for cutting corn" , "Tool for cutting grain" , "Farmer's tool" .)