Dogs, about fifty -- they have arrived in new homes (8)
I believe the answer is:
settlers
'have arrived in new homes' is the definition.
I can't judge whether this defines the answer.
'dogs about fifty they' is the wordplay.
'dogs' becomes 'setters' (setter is a kind of dog).
'about' means one lot of letters goes inside another.
'fifty they' becomes 'l' (I can't explain this - if you can you should believe this answer much more).
'setters' placed around 'l' is 'SETTLERS'.
Can you help me to learn more?
(Other definitions for settlers that I've seen before include "Colonists" , "Trestles for pioneers" , "who pay their bills?" , "Immigrants" , "Pioneers, homesteaders" .)