Colonist gives a decisive blow (7)
I believe the answer is:
settler
I believe this is a double definition.
'colonist' is the first definition.
(I know that colonist can be written as settler)
'a decisive blow' is the second definition.
(I've seen this before)
'gives' is the link.
(Other definitions for settler that I've seen before include "who pays his bills" , "Coloniser - arbitrator" , "One taking up residence in a new area" , "New colonist" , "Early mover into an area" .)