First in space (7)
I believe the answer is:
opening
I believe this clue is a triple definition!
'first' is the first definition.
('opening' can be similar in meaning to 'first')
'in' is the second definition.
(thesaurus)
'space' is the third definition.
(opening is a kind of space)
(Other definitions for opening that I've seen before include "Vacancy" , "First -- beginning" , "Inauguration" , "premiere" , "Orifice; opportunity" .)