Make arrangements for alien to leave Mars, perhaps (4)
I believe the answer is:
plan
'make arrangements' is the definition.
(thesaurus)
'alien to leave mars perhaps' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'to leave' is a deletion indicator.
'mars perhaps' becomes 'planet' (mars is a kind of planet).
'planet' with 'et' taken away is 'PLAN'.
'for' acts as a link.
(Other definitions for plan that I've seen before include "Schedule" , "Blueprint" , "Formulate" , "Arrangement" , "Programme" .)