Lower half of floor disappearing is concerning (4)
I believe the answer is:
asto
'disappearing is concerning' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'lower half of floor' is the wordplay.
'lower' becomes 'ast' (I can't justify this - if you can you should give a lot more credence to this answer).
'half of' suggests halving the letters.
'floor' becomes 'ko' (I've seen this in other clues).
'ko' cut in half is 'o'.
'ast'+'o'='ASTO'
Can you help me to learn more?
(Another definition for asto that I've seen is " about...".)