Pachyderm (10)
'pachyderm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for elephant that I've seen before include "Large animal found in the plane?" , "Large pachyderm" , "Endangered Species" , "Trunked mammal" , "The symbol of the Republican Party" .)
(Other definitions for mastodon that I've seen before include "Prehistoric creature" , "Large mammal" , "giant from the past" , "Extinct elephant-like mammal" , "Big beast" .)
(Other definitions for hippopotamus that I've seen before include "Creature" , "Large African mammal frequenting rivers" , "Large semi-aquatic mammal" , "Thick-skinned herbivorous animal of tropical Africa" , "African river beast" .)
'pachyderm' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rhinoceros that I've seen before include "Large horned animal" , "Ionesco play" , "Native of Africa" , "Ionesco's work" , "Pachyderm" .)