A girl that is with a dog (6)
I believe the answer is:
lassie
'dog' is the definition.
(I've seen this before)
'a girl that is' is the wordplay.
'a girl' becomes 'lass' (lass can informally mean a girl).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'lass'+'ie'='LASSIE'
'with a' is the link.
(Other definitions for lassie that I've seen before include "TV series collie dog" , "Canine film star" , "girl found" , "Hollywood film-star dog" , "TV/film dog" .)