Inexperienced, not beginning to attempt part of space mission (2-5)
'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'.
(Other definitions for re-entry that I've seen before include "Hitting earth's atmosphere again", "A spacecraft's track coming back into atmosphere", "Spacecraft's return to earth", "Spacecraft's return phase", "Return of a spacecraft to the earth's atmosphere".)