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' means one should take alternating letters.
The alternate letters of 'ideal' are 'iel'.
'is' is the link.
(Other definitions for spaniel that I've seen before include "Dog with long silky coat", "Droopy-eared dog", "Droopy-ear dog", "Cocker - -", "See 1 Across", "Dog found among the alpines", "See 13", "Springer- - -", "[HOUND]" and "Kind of dog, may be a cocker".)