Neil Armstrong's first up from another world (5)
I believe the answer is:
alien
'from another world' is the definition.
(I've seen this in another clue)
'neil armstrong's first up' is the wordplay.
'armstrong's first' becomes 'a' (1st letter of 'armstrong').
'up' is a reversal indicator (in down clue: letters go upwards).
'neil'+'a'='neila'
'neila' in reverse letter order is 'ALIEN'.
(Other definitions for alien that I've seen before include "ET" , "Not native" , "Foreign (from outer space?)" , "Otherworldly" , "of foreigners" .)