Baby, age about 1, back at home (9)
I believe the answer is:
miniature
'baby' is the definition.
(thesaurus)
'age about 1 back at home' is the wordplay.
'age' becomes 'mature' (synonyms).
'about' indicates putting letters inside.
'm' becomes 'I' (Roman numeral).
'back' says the letters should be written in reverse.
'at home' becomes 'in' (similar in meaning).
'in' backwards is 'ni'.
'i'+'ni'='ini'
'mature' enclosing 'ini' is 'MINIATURE'.
(Other definitions for miniature that I've seen before include "I ruminate (anag)" , "Smaller than normal" , "illuminated letter" , "Of a much smaller size than usual" , "Putting game" .)