At home, plump with new child (6)
I believe the answer is:
infant
'child' is the definition.
(I know that infant is a type of child)
'at home plump with new' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'plump' becomes 'fat' (synonyms).
'with' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'fat' placed around 'n' is 'fant'.
'in'+'fant'='INFANT'
(Other definitions for infant that I've seen before include "Thing in early stage; minor (legal)" , "Tot" , "Small baby" , "Wee one" , "Rudimentary" .)