One travelling to Saturn? (9)
I believe the answer is:
astronaut
'one travelling to saturn?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one travelling to saturn?' is the wordplay.
'one' becomes 'a'.
'travelling' indicates anagramming the letters.
'to'+'saturn'='tosaturn'
'tosaturn' with letters rearranged gives 'stronaut'.
'a'+'stronaut'='ASTRONAUT'
(Other definitions for astronaut that I've seen before include "Buzz Aldrin, for example" , "Santa tour for space traveller" , "Space voyager" , "Person trained to travel in a spacecraft" , "Space explorer" .)