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 "Type of dog", "Leaps in (anag.)", "Dog with long silky coat", "See 1 Across", "See 13", "Gundog; obsequious type", "Dog found among the alpines", "Alpines (anag)", "Dog - leaps in (anag)" and "Long-eared dog".)