Dog William's regularly taken westwards across country (6)
I believe the answer is:
saluki
'dog' is the definition.
(Saluki is an example)
'william's regularly taken westwards across country' is the wordplay.
'regularly taken' means one should take alternating letters (letters taken at regular intervals).
'westwards' shows that the letters should be reversed in order (letters go right to left or from 'east' to 'west').
'across' means one lot of letters goes inside another.
'country' becomes 'UK' (United Kingdom).
The alternate letters of 'williams' are 'ilas'.
'ilas' in reverse letter order is 'sali'.
'sali' enclosing 'uk' is 'SALUKI'.
(Other definitions for saluki that I've seen before include "Tall slender dog" , "Gazelle-hound" , "Tall hunting dog (like a greyhound) from Egypt and south-western Asia" , "Persian greyhound" , "Dog resembling a greyhound" .)