Pioneer, crossword compiler, holding line (7)
I believe the answer is:
settler
'pioneer' is the definition.
(I've seen this before)
'crossword compiler holding line' is the wordplay.
'crossword compiler' becomes 'setter' (I've seen this before).
'holding' is an insertion indicator.
'line' becomes 'l'.
'setter' placed around 'l' is 'SETTLER'.
(Other definitions for settler that I've seen before include "New colonist" , "new inhabitant" , "Person who moves into an uninhabited country" , "One taking up residence in a new area" , "who pays his bills" .)