Astronauts primarily vote by rating missions (7)
I believe the answer is:
apollos
'missions' is the definition.
(Apollo moon missions)
'astronauts primarily vote by rating' is the wordplay.
'primarily' indicates taking the first letters.
'vote' becomes 'poll' (polling is a kind of voting).
'by' says to put letters next to each other.
'rating' becomes 'OS' (Ordinary Seaman - rating can mean a sailor).
The initial letter of 'astronauts' is 'a'.
'a'+'poll'+'os'='APOLLOS'
(Other definitions for apollos that I've seen before include "striking lads" , "Butterflies" .)