Body in space time to arrive first (5)
I believe the answer is:
comet
'body in space' is the definition.
(I've seen this before)
'time to arrive first' is the wordplay.
'time' becomes 't' (abbreviation).
'to arrive' becomes 'come' (coming is a kind of arriving).
'first' means one lot of letters go next to another.
't' put after 'come' is 'COMET'.
(Other definitions for comet that I've seen before include "potential space invader" , "Celestial body with luminous tail" , "bright star" , "Halley's space traveller?" , "one of Santa's reindeer" .)