Home, a chap's castle ultimately (5)
I believe the answer is:
abode
'home' is the definition.
('abode' can be a synonym of 'home')
'a chap's castle ultimately' is the wordplay.
'chap' becomes 'bod' (I've seen this in other clues**).
'ultimately' suggests the final letters.
The final letter of 'castle' is 'e'.
'a'+'bod'+'e'='ABODE'
(Other definitions for abode that I've seen before include "Archaic dwelling place" , "Dwelling, residence" , "Home or domicile" , "Pad" , "House, home" .)