Mean alien leaves Earth, perhaps (4)
I believe the answer is:
plan
'mean' is the definition.
(planning is a kind of meaning)
'alien leaves earth perhaps' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'leaves' suggests deleting specific letters.
'earth perhaps' becomes 'planet' (Earth is an example).
'planet' with 'et' taken away is 'PLAN'.
(Other definitions for plan that I've seen before include "Intend" , "Project" , "Detailed proposal" , "chart" , "Aim" .)