Colonist's dog crossing lake (7)
I believe the answer is:
settler
'colonist's' is the definition.
(synonyms)
'dog crossing lake' is the wordplay.
'dog' becomes 'setter' (setter is a type of dog).
'crossing' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'setter' going around 'l' is 'SETTLER'.
(Other definitions for settler that I've seen before include "Person who moves into an uninhabited country" , "Coloniser - arbitrator" , "Early colonist" , "newcomer, perhaps" , "Early mover into an area" .)