I go through the sales breakdown with her (6)
I believe the answer is:
lassie
'i go' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'the sales breakdown with her' is the wordplay.
'breakdown' indicates an anagram.
'with' is an insertion indicator.
'her' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'sales' is an anagram of 'lasse'.
'lasse' placed around 'i' is 'LASSIE'.
'through' is the link.
Can you help me to learn more?
(Other definitions for lassie that I've seen before include "Collie featuring in many films" , "Film and TV dog" , "TV/film dog" , "Girl from Scotland and dog from film" , "Young Scottish female" .)