Dog has left pioneer (7)
I believe the answer is:
settler
'pioneer' is the definition.
(I've seen this before)
'dog has left' is the wordplay.
'dog' becomes 'setter' (setter is a type of dog).
'has' means one lot of letters goes inside another.
'left' becomes 'l' (common abbreviation).
'setter' enclosing 'l' is 'SETTLER'.
(Other definitions for settler that I've seen before include "One colonising new area" , "New colonist" , "who pays his bills" , "Coloniser - arbitrator" , "decisive blow" .)