Martians at last on flying saucer — that's deliberate (7)
I believe the answer is:
discuss
'deliberate' is the definition.
(to discuss something is to deliberate it)
'martians at last on flying saucer' is the wordplay.
'at last' says to take the final letters.
'on' means one lot of letters go next to another.
'flying saucer' becomes 'discus' (disc made to 'fly' when thrown).
The last letter of 'martians' is 's'.
's' put after 'discus' is 'DISCUSS'.
'that's' acts as a link.
(Other definitions for discuss that I've seen before include "Go into" , "Verbally T [TREAT]" , "Debate" , "Settle" , "Confer" .)