Small animal in Post Office? (5)

I believe the answer is:

hippo

I'm a little stuck... Click here to teach me more about this clue!

'small animal' is the definition.
'hippo' can be an answer for 'animal' (I've seen this before). I am not certain of the 'small' bit.

'in post office?' is the wordplay.
'in' becomes 'hip' (both can mean fashionable).
'post office?' becomes 'po'.
'hip'+'po'='HIPPO'

Can you help me to learn more?

(Other definitions for hippo that I've seen before include "Tropical African mud wallower" , "Aquatic creature" , "Thick-skinned type" , "Large semi-aquatic mammal (abbr)" , "Wallowing creature" .)

I've seen this clue in The Hindu.

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