Dog is cross; oddly ideal (7)
I believe the answer is:
Here is my best explanation:
'dog' is the definition.
(spaniel is a kind of dog)
'cross oddly ideal' is the subsidiary indication.
'cross' becomes 'span' ('span' can be a synonym of 'cross'**).
'oddly' indicates alternate letters.
The alternate letters of 'ideal' are 'iel'.
'is' is the link.
(Other definitions for spaniel that I've seen before include "Dog with drooping ears", "See 13", "See 1 Across", "Long-eared dog", "Droopy-eared dog", "Kind of dog, may be a cocker", "Springer- - -", "Alpines (anag)", "Gundog; obsequious type" and "Leaps in (anag.)".)