Long dull passage covered in loose rock, apparently (6)
I believe the answer is:
screed
'long dull passage' is the definition.
'covered in loose rock apparently' is the wordplay.
'covered in' says to put letters next to each other (some letters go on top of or cover others).
'loose rock' becomes 'scree' (I've seen this before).
'apparently' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'scree' put next to 'd' is 'SCREED'.
Can you help me to learn more?
(Other definitions for screed that I've seen before include "Lengthy piece of writing" , "Thin layer of mortar" , "extensive text" , "Mortar used for levelling" , "A long written effusion" .)