Almost appalling layer is less clean (7)
I believe the answer is:
dirtier
'less clean' is the definition.
('dirty' can be the opposite of 'clean' so perhaps 'dirtier' is reasonable for 'less clean')
'almost appalling layer' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'appalling' becomes 'dire' (I've seen this in another clue).
'layer' becomes 'tier' (tier is a kind of layer).
'dire' with its last letter taken off is 'dir'.
'dir'+'tier'='DIRTIER'
'is' is the link.
(Other definitions for dirtier that I've seen before include "Filthier" , "Less clean" , "More filthy" , "cleaner? The opposite" , "More grubby" .)