Creatures that are party to a terrible crime (7)
I believe the answer is:
dormice
'creatures' is the definition.
(dormouse is a kind of creature)
'are party to a terrible crime' is the wordplay.
'are party' becomes 'do'.
'to a terrible' is an anagram indicator.
'crime' with letters rearranged gives 'rmice'.
'do'+'rmice'='DORMICE'
'that' is the link.
(Other definitions for dormice that I've seen before include "Mammals" , "Little hibernating rodents" , "little things?" , "Furry animals" , "Bushy-tailed rodents" .)