Animal in its smaller form, having no tail! (5)
I believe the answer is:
rhino
'animal' is the definition.
(short for rhinoceros)
'in its smaller form having no tail' is the wordplay.
'in' is an insertion indicator.
'its smaller' becomes 'rho' (I am not sure about this - if you are sure you should believe this answer much more).
'form' becomes 'ink' (I have seen 'form of writing' mean 'ink' so perhaps 'form' could also mean 'ink').
'having no tail' means to remove the last letter (I've seen 'has no tail' mean this).
'ink' with its final letter taken off is 'in'.
'rho' enclosing 'in' is 'RHINO'.
Can you help me to learn more?
(Other definitions for rhino that I've seen before include "Large animal (abbr) - money (slang)" , "Funds" , "Large animal with a horn on its snout, in short" , "one may charge unwary tourist?" , "Thick-skinned animal (abbr) - money (slang)" .)