Dog that is pursuing Antarctic explorer (7)
I believe the answer is:
scottie
'dog' is the definition.
(Scottie is an example)
'that is pursuing antarctic explorer' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'pursuing' means one lot of letters go next to another.
'antarctic explorer' becomes 'scott' (Robert Falcon Scott, Scott of the Antarctic).
'ie' put after 'scott' is 'SCOTTIE'.
(Other definitions for scottie that I've seen before include "Type of terrier" , "Breed of terrier" , "Dog" , "animal" .)