Completely at home with Dorothy's dog (2,4)
I believe the answer is:
in toto
'completely' is the definition.
(I've seen this before)
'home with dorothy's dog' is the wordplay.
'home' becomes 'in' (to be in is to be (at) home).
'with' means one lot of letters go next to another.
'dorothy's dog' becomes 'toto' (Dorothy's dog in The Wizard of Oz).
'in'+'toto'='IN TOTO'
'at' is the link.
(Other definitions for in toto that I've seen before include "Completely - entirely (Latin)" , "In entirety" , "Wholly" , "overall" , "Tranquillity" .)