Crazy origin of rather popular spaceman (7)
I believe the answer is:
gagarin
'spaceman' is the definition.
(I have seen 'old spaceman' mean 'gagarin' so perhaps 'spaceman' could also mean 'gagarin')
'crazy origin of rather popular' is the wordplay.
'crazy' becomes 'gaga' (I've seen this in another clue).
'origin of' says to take the initial letters.
'popular' becomes 'in' ('in' can mean trendy).
The initial letter of 'rather' is 'r'.
'gaga'+'r'+'in'='GAGARIN'
(Other definitions for gagarin that I've seen before include "Yuri --, cosmonaut" , "Astronaut" , "circumnavigator" , "Yuri ......, first Russian into space" , "Yuri . . . . . . . made the first manned space flight" .)