Flat entrance partly concealed (6)
I believe the answer is:
Here is my best explanation:
'concealed' is the definition.
'flat entrance partly' is the subsidiary indication.
'partly' indicates a hidden word.
'latent' is hidden in the letters of 'flat entrance'.
(Other definitions for latent that I've seen before include "Hidden, dormant", "Not presently active", "Hidden, not manifest", "Hidden but able to be developed", "Existing but not yet active, say talent", "Existing but hidden", "Dormant, hidden", "Existing but not yet active or visible", "Not yet visible or developed" and "Talent (anag)".)