Slavery deforms terribly (7)
I believe the answer is:
serfdom
'slavery' is the definition.
(I know that Serfdom is a type of slavery)
'deforms terribly' is the wordplay.
'terribly' is an anagram indicator.
'deforms' is an anagram of 'SERFDOM'.
(Other definitions for serfdom that I've seen before include "Feudal worker's state" , "State of slavery" , "Feudal labour" .)