The ram's terrible as a pet (7)
I believe the answer is:
hamster
'a pet' is the definition.
(I've seen this before)
'the ram's terrible' is the wordplay.
'terrible' is an anagram indicator.
'the'+'rams'='therams'
'therams' is an anagram of 'HAMSTER'.
'as' is the link.
(Other definitions for hamster that I've seen before include "Short-tailed burrowing rodent" , "Animal" , "Furry pet" , "Oddly, the arms of a rodent" , "Kind of rodent with cheekpouches, often a pet" .)