Shampoo pup — it could become a new animal! (12)
I believe the answer is:
hippopotamus
'a new animal' is the definition.
'hippopotamus' can be an answer for 'animal' (thesaurus). I'm not sure about the 'a new' bit.
'shampoo pup it could become' is the wordplay.
'could become' indicates an anagram.
'shampoo'+'pup'+'it'='shampoopupit'
'shampoopupit' is an anagram of 'HIPPOPOTAMUS'.
(Other definitions for hippopotamus that I've seen before include "Large thick-skinned herbivore" , "Large semi-aquatic mammal" , "African animal" , "Large aquatic beast" , "Oppo hams it up (anag)" .)