Worried if aliens settle (8)
I believe the answer is:
finalise
'settle' is the definition.
(I know that settle can be written as finalise)
'worried if aliens' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'if'+'aliens'='ifaliens'
'ifaliens' is an anagram of 'FINALISE'.
(Other definitions for finalise that I've seen before include "Agree on definitive version of something" , "Put the last touches to" , "Put an end to, complete" , "settle" , "Conclude" .)