At home in America's capital, say (8)
I believe the answer is:
astatine
'at' is the definition.
(synonyms)
'home in america's capital say' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'in' indicates putting letters inside.
'capital' indicates taking the first letters.
'say' becomes 'state' (synonyms).
The initial letter of 'americas' is 'a'.
'a'+'state'='astate'
'in' going into 'astate' is 'ASTATINE'.
(Other definitions for astatine that I've seen before include "radioactive material" , "Rare element" , "Radioactive element" , "Chemical element At" , "Atomic number 85" .)