Terrible housing rated badly, head off (9)
I believe the answer is:
sidetrack
'head off' is the definition.
(thesaurus)
'terrible housing rated badly' is the wordplay.
'terrible' becomes 'sick' (I am not sure about this - if you are sure you should believe this answer much more).
'housing' means one lot of letters goes inside another.
'badly' indicates anagramming the letters (letters in the wrong order).
'rated' with letters rearranged gives 'detra'.
'sick' enclosing 'detra' is 'SIDETRACK'.
Can you help me to learn more?
(Other definitions for sidetrack that I've seen before include "Distract (from big issue)" , "small way" , "draw away" , "Divert" , "lead away" .)