The ram's upset pet (7)
I believe the answer is:
hamster
'pet' is the definition.
(I've seen this before)
'the ram's upset' is the wordplay.
'upset' indicates an anagram.
'the'+'rams'='therams'
'therams' with letters rearranged gives 'HAMSTER'.
(Other definitions for hamster that I've seen before include "Little beast" , "Rodent often kept as pet" , "Kind of rodent with cheekpouches, often a pet" , "Furry pet" , "one running round in circles?" .)