Decorum in dirty hovel, way above (7)
I believe the answer is:
modesty
'decorum' is the definition.
(I know that decorum can be written as modesty)
'dirty hovel way above' is the wordplay.
'dirty hovel' becomes 'sty' (as in a pig sty).
'way' becomes 'mode' (synonyms).
'above' means one lot of letters go next to another (in a down clue, some letters go above others).
'sty' put after 'mode' is 'MODESTY'.
'in' is the link.
(Other definitions for modesty that I've seen before include "Reserve" , "Unassuming character" , "Unassuming quality" , "Unpretentious style" , "Diffidence" .)