NASA lunar program (6)
I believe the answer is:
apollo
'nasa lunar program' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for apollo that I've seen before include "Greek god; large European butterfly" , "Greek god - US space programme dedicated to moon landings - butterfly" , "US moon landing programme" , "Lad who's striking" , "Greek sun god and leader of the Muses" .)