Unit stops landing spacecraft (7)
I believe the answer is:
pioneer
'spacecraft' is the definition.
(I've seen this before)
'unit stops landing' is the wordplay.
'unit' becomes 'one' ('one' can be a synonym of 'unit'**).
'stops' means one lot of letters goes inside another.
'landing' becomes 'pier' (pier is a kind of landing).
'one' put inside 'pier' is 'PIONEER'.
(Other definitions for pioneer that I've seen before include "Colonist" , "make the first move!" , "An explorer in new territory" , "true original" , "Explorer - innovator" .)