I retain unusual characteristic of sloth (7)
I believe the answer is:
inertia
'characteristic of sloth' is the definition.
'inertia' can be an answer for 'sloth' (inertia can mean laziness or sloth). I am not sure about the 'characteristic of' bit.
'i retain unusual' is the wordplay.
'unusual' indicates anagramming the letters.
'retain' anagrammed gives 'nertia'.
'i'+'nertia'='INERTIA'
Can you help me to learn more?
(Other definitions for inertia that I've seen before include "inclination not to budge" , "Chemical or other inactivity" , "Feeling of slowness to act, as I retain" , "Passivity" , "Stagnating state" .)