Pet with a hard master training it (7)
I believe the answer is:
hamster
'pet' is the definition.
(I've seen this before)
'a hard master training it' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'master' becomes 'm'.
'training' indicates anagramming the letters (to train can mean to grow something into a shape).
'it' becomes 'ster' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'h'+'m'='ahm'
'ahm' with letters rearranged gives 'ham'.
'ham'+'ster'='HAMSTER'
'with' is the link.
Can you help me to learn more?
(Other definitions for hamster that I've seen before include "one running round in circles?" , "Oddly, the arms of a rodent" , "Short-tailed burrowing rodent" , "Animal" , "Common pet rodent" .)