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' (do can informally mean a party).
'to a terrible' indicates anagramming the letters.
'crime' anagrammed gives 'rmice'.
'do'+'rmice'='DORMICE'
'that' acts as a link.
(Other definitions for dormice that I've seen before include "Mammals" , "Little hibernating rodents" , "little things?" , "Furry animals" , "Bushy-tailed rodents" .)