Jetty contains single colonist (7)
I believe the answer is:
pioneer
'colonist' is the definition.
(I know that pioneer is a type of colonist)
'jetty contains single' is the wordplay.
'jetty' becomes 'pier' (I've seen this in another clue).
'contains' means one lot of letters goes inside another.
'single' becomes 'one' (a single thing - only one).
'pier' enclosing 'one' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "Innovate" , "One who blazes the trail for others" , "Innovator; first settler" , "make the first move!" , "Original developer" .)