So a truant could become a spaceman? (9)
I believe the answer is:
astronaut
'a spaceman?' is the definition.
('astronaut' can be a synonym of 'spaceman')
'so a truant could become' is the wordplay.
'could become' indicates anagramming the letters.
'so'+'a'+'truant'='soatruant'
'soatruant' anagrammed gives 'ASTRONAUT'.
(Other definitions for astronaut that I've seen before include "Armstrong or Sharman, say" , "Buzz Aldrin, for example" , "Skier" , "Buzz Aldrin, e.g." , "High-flyer" .)