Stop larking about and colonise a treeless upland (6,4)
I believe the answer is:
settle down
'colonise a treeless upland' is the definition.
'settle down' can be an answer for 'colonise' (thesaurus). I am not certain of the 'a treeless upland' bit.
'stop larking about and' is the wordplay.
'stop' becomes 'settle' (settling is a kind of stopping).
'larking about' becomes 'dow' (I can't justify this - if you can you should believe this answer much more).
'and' becomes 'n' (common abbreviation for 'and').
'settle'+'dow'+'n'='SETTLE DOWN'
Can you help me to learn more?
(Other definitions for settle down that I've seen before include "Become quiet and orderly" , "put down roots" .)