Hard master punished pet (7)
I believe the answer is:
hamster
'pet' is the definition.
(I've seen this before)
'hard master punished' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'punished' indicates an anagram.
'master' is an anagram of 'amster'.
'h'+'amster'='HAMSTER'
(Other definitions for hamster that I've seen before include "Kind of rodent with cheekpouches, often a pet" , "The arms (anag)" , "Rodent with large cheek pouches" , "Rodent often kept as pet" , "Common pet rodent" .)