The dog leaps in madly (7)
I believe the answer is:
spaniel
'the dog' is the definition.
(spaniel is a kind of dog)
'leaps in madly' is the wordplay.
'madly' indicates an anagram.
'leaps'+'in'='leapsin'
'leapsin' is an anagram of 'SPANIEL'.
(Other definitions for spaniel that I've seen before include "Dog - leaps in (anag)" , "See 1 Across" , "Alpines (anag)" , "Droopy-eared dog" , "Animal" .)