Graduates have medium to small reptiles (6)
I believe the answer is:
mambas
'reptiles' is the definition.
(mamba is a kind of reptile)
'graduates have medium to small' is the wordplay.
'graduates' becomes 'MAs' (people with Master of Arts degrees).
'have' indicates putting letters inside.
'medium' becomes 'm' (as in UK clothing sizes).
'to small' becomes 'ba' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'm'+'ba'='mba'
'mas' placed around 'mba' is 'MAMBAS'.
Can you help me to learn more?
(Other definitions for mambas that I've seen before include "these snakes bite dangerously" , "Arboreal snakes" .)