Space programme without leading actor is a real slog! (4)
I believe the answer is:
trek
'slog' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for trek that I've seen before include "Long and difficult journey" , "Tramp" , "Long difficult walk" , "Long walk in the mountains?" , "A long arduous journey as in S Africa" .)