Animal -- one very quiet in house (5)
I believe the answer is:
hippo
'animal' is the definition.
(I've seen this before)
'one very quiet in house' is the wordplay.
'one' becomes 'i' (Roman numeral).
'very quiet' becomes 'pp'.
'in' is an insertion indicator.
'house' becomes 'ho' (abbreviation for house).
'i'+'pp'='ipp'
'ipp' inserted into 'ho' is 'HIPPO'.
(Other definitions for hippo that I've seen before include "One wallows in mud" , "Large semi-aquatic pachyderm (abbr)" , "Water-loving mammal" , "large African creature" , "African wallower (abbr)" .)