Someday perhaps will be this dog's day (7)
I believe the answer is:
samoyed
'this dog's day' is the definition.
'Samoyed' can be an answer for 'dog's' (Samoyed is an example). I'm not sure about the remainder of the definition.
'someday perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'someday' anagrammed gives 'SAMOYED'.
'will be' is the link.
Can you help me to learn more?
(Other definitions for samoyed that I've seen before include "Sled dog" , "Breed of white dog from Siberia - May's ode (anag)" , "Arctic dog" , "Some day (anag) - dog (from Siberia?)" , "[HOUND] ?" .)