Completely at home, toddler with duck (2,4)
I believe the answer is:
in toto
'completely' is the definition.
(thesaurus)
'home toddler with duck' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'toddler' becomes 'tot' ('tot' can be a synonym of 'toddler').
'with' says to put letters next to each other.
'duck' becomes 'o'.
'in'+'tot'+'o'='IN TOTO'
'at' is the link.
(Other definitions for in toto that I've seen before include "As a whole (Lat.)" , "Completely - entirely (Latin)" , "In entirety" , "Wholly" , "As a whole (Latin)" .)