Disreputable, heartless lowlife admitting this is agony (6)
I believe the answer is:
louche
'disreputable' is the definition.
(similar in meaning)
'heartless lowlife admitting this is agony' is the wordplay.
'heartless' means to remove the middle letters.
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'this is agony' becomes 'ouch' (I can't justify this - if you can you should give a lot more credence to this answer).
'lowlife' with its centre taken out is 'le'.
'le' enclosing 'ouch' is 'LOUCHE'.
Can you help me to learn more?
(Other definitions for louche that I've seen before include "Ignominious" , "Rakishly disreputable" , "Shasy or questionable, from France" , "Questionable or shady, from French" , "Shifty or disreputable" .)