Large dog walks in regularly with terriers (5)
I believe the answer is:
akita
'large dog' is the definition.
(breed of dog)
'walks in regularly with terriers' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'with' is a charade indicator (letters next to each other).
'terriers' becomes 'ta' (nickname for Territorial Army members).
'walks'+'in'='walksin'
The alternating letters of 'walksin' are 'aki'.
'aki'+'ta'='AKITA'
(Other definitions for akita that I've seen before include "powerful dog" , "dog from Japan" , "Japanese breed of dog" , "heavy brute?" .)