Dog crossing line is pioneer (7)
I believe the answer is:
settler
'is pioneer' is the definition.
(I've seen this in another clue)
'dog crossing line' is the wordplay.
I cannot really understand how this works, but
'line' could be 'l' (used when specifying particular lines from a poem) and 'l' is found in the answer.
'dog' could be 'setter' (setter is a type of dog) and 'setter' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for settler that I've seen before include "Early colonist" , "Colonist, pioneer" , "Early mover into an area" , "decisive blow" , "new inhabitant" .)