Rescue dog that is with northern girl (6)
I believe the answer is:
lassie
'rescue dog' is the definition.
'that is with northern girl' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'with' means one lot of letters go next to another.
'northern girl' becomes 'lass' (lass can informally mean a girl. I am not sure about the 'northern' bit.).
'ie' after 'lass' is 'LASSIE'.
Can you help me to learn more?
(Other definitions for lassie that I've seen before include "Movie role for several dog stars" , "Cinema dog who went home" , "Scottish girl - canine movie star" , "Screen mutt - Scottish girl" , "Screen canine" .)