At home within a say... (8)
I believe the answer is:
astatine
'at' is the definition.
(synonyms)
'home within a say' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'within' is an insertion indicator.
'say' becomes 'state' (synonyms).
'a'+'state'='astate'
'in' put into 'astate' is 'ASTATINE'.
(Other definitions for astatine that I've seen before include "Chemical element At" , "Atomic number 85" , "Rare element" , "Radioactive element" , "radioactive material" .)