Cross that is large breed of dog (7)
I believe the answer is:
Here is my best explanation:
'breed of dog' is the definition.
(I've seen this before)
'cross that is large' is the wordplay.
'cross' becomes 'span' ('span' can be a synonym of 'cross'**).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'large' becomes 'l'.