What launches spacemen do in craft (5)
I believe the answer is:
skill
'craft' is the definition.
(a craft is a skill or talent)
'what launches spacemen do in' is the wordplay.
'what launches' indicates taking the first letters (the letter that begins or launches the word).
'do in' becomes 'kill' (do in can mean to assassinate or kill).
The first letter of 'spacemen' is 's'.
's'+'kill'='SKILL'
(Other definitions for skill that I've seen before include "Craft; dexterity" , "Talent" , "Prowess" , "Knack" , "Ability - dexterity" .)