Terrible rare condition stops choir working (8)
I believe the answer is:
horrific
'terrible' is the definition.
(synonyms)
'rare condition stops choir working' is the wordplay.
'rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations).
'condition' becomes 'if' (I've seen this in other clues).
'stops' indicates putting letters inside.
'working' indicates anagramming the letters.
'r'+'if'='rif'
'choir' is an anagram of 'horic'.
'rif' placed into 'horic' is 'HORRIFIC'.
(Other definitions for horrific that I've seen before include "Dire" , "Exciting intense fear and repugnance" , "Appalling" , "Frightful" , "Causing fear" .)