Clue: Corridor covered with it — rising fungus and lichen, say (6)
Answer: THALLI
Explanation: Definition: fungus and lichen, say
 HALL(a corridor/a passageway just inside the entrance of a house) **contained in**(covered with) **reversal of**(… — rising, in a down clue) IT.
Defn: Plant bodies that show no distinct parts such as stems, leaves and roots, examples of which/say, are fungus and lichen.