Help, reportedly, after large slab impedes their progress (8)
I believe the answer is:
blockade
'impedes their progress' is the definition.
I can't tell whether this defines the answer.
'help reportedly after large slab' is the wordplay.
'help' becomes 'aide' (aide is a kind of help).
'reportedly' indicates a 'sounds like' (homophone) clue.
'after' says to put letters next to each other.
'large slab' becomes 'block' (I am not sure about this - if you are sure you should believe this answer much more).
'aide' is a homophone of 'ade'.
'ade' put after 'block' is 'BLOCKADE'.
Can you help me to learn more?
(Other definitions for blockade that I've seen before include "Military restriction of access" , "The sealing off a place to prevent entry or exist" , "Seal off (a sea port)" , "Prevent(ion of) access and exit" , "clampdown" .)