Inexperienced, not beginning to attempt part of space mission (2-5)
I believe the answer is:
re-entry
'part of space mission' is the definition.
(re-entry of a spacecraft into Earth's atmosphere)
'inexperienced not beginning to attempt' is the wordplay.
'inexperienced' becomes 'green' (similar in meaning).
'not beginning' means to remove the first letter.
'to attempt' becomes 'try' (synonyms).
'green' with its initial letter removed is 'reen'.
'reen'+'try'='RE-ENTRY'
(Other definitions for re-entry that I've seen before include "A spacecraft's track coming back into atmosphere" , "Return of a spacecraft to the earth's atmosphere" , "Hitting earth's atmosphere again" , "Spacecraft's return to earth" , "Return from outer space" .)