Length oddly ideal for this dog (7)
I believe the answer is:
spaniel
'this dog' is the definition.
(spaniel is a kind of dog)
'length oddly ideal' is the wordplay.
'length' becomes 'span' (I've seen this in another clue).
'oddly' means one should take alternating letters (letters in odd-numbered positions).
The alternate letters of 'ideal' are 'iel'.
'span'+'iel'='SPANIEL'
'for' is the link.
(Other definitions for spaniel that I've seen before include "Alpines (anag)" , "See 1 Across" , "Gundog; obsequious type" , "Type of dog" , "Dog found among the alpines" .)