Type of hippo or marmoset (5)
I believe the answer is:
pygmy
This clue is a double definition.
'type of hippo' is the first definition.
(I've seen this in another clue)
'marmoset' is the second definition.
(pygmy marmoset is a kind of marmoset)
(Other definitions for pygmy that I've seen before include "Diminutive person" , "Member of small African people" , "Short-statured human" , "Of very small stature" , "Homunculus" .)