Completely at home, having had enough to eat (2,4)
I believe the answer is:
in full
'completely' is the definition.
(in full means completely or totally)
'home having had enough to eat' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'having' means one lot of letters go next to another.
'had enough to eat' becomes 'full'.
'in'+'full'='IN FULL'
'at' is the link.
(Other definitions for in full that I've seen before include "Tobacco constituent" , "Completely" , "The entire amount, with nothing omitted" .)