Corridor covered with it — rising fungus and lichen, say (6)
I believe the answer is:
thalli
'fungus and lichen say' is the definition.
I can't tell whether this definition defines the answer.
'corridor covered with it rising' is the wordplay.
'corridor' becomes 'hall' (hall is a kind of corridor).
'covered with' indicates putting letters inside.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'it' backwards is 'ti'.
'hall' put within 'ti' is 'THALLI'.
Can you help me to learn more?
(Another definition for thalli that I've seen is " Plant bodies".)