Flat entrance conceals something hidden (6)
I believe the answer is:
latent
'hidden' is the definition.
(latent can mean hidden or unused)
'flat entrance conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'LATENT' is hidden in the letters of 'flat entrance'.
'something' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for latent that I've seen before include "As yet undeveloped" , "Not presently active" , "Inactive - potential" , "Not yet visible or developed" , "Unrealised, potential" .)