The beast starts to be hopping mad! (5)
I believe the answer is:
hippo
'the beast' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hippo that I've seen before include "one that's large and fleshy" , "large African creature" , "Wallowing creature" , "Augustine's place" , "African river mammal" .)