Pub blasted into space? That's not the main story (7)
I believe the answer is:
subplot
'not the main story' is the definition.
(I've seen this before)
'pub blasted into space?' is the wordplay.
'blasted' indicates anagramming the letters.
'into' means one lot of letters goes inside another.
'space?' becomes 'slot' (I can't explain this - if you can you should believe this answer much more).
'pub' with letters rearranged gives 'ubp'.
'ubp' going within 'slot' is 'SUBPLOT'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for subplot that I've seen before include "By-line, perhaps" , "Storyline within a storyline" , "not the main story" .)