Leaps in, surprising the dog (7)
I believe the answer is:
spaniel
'the dog' is the definition.
(spaniel is a kind of dog)
'leaps in surprising' is the wordplay.
'surprising' is an anagram indicator.
'leaps'+'in'='leapsin'
'leapsin' is an anagram of 'SPANIEL'.
(Other definitions for spaniel that I've seen before include "See 13" , "Gundog; obsequious type" , "Droopy-eared dog" , "Type of dog" , "Kind of dog, may be a cocker" .)