Fashion house's finale, dull (4)
I believe the answer is:
fade
'dull' is the definition.
(thesaurus)
'fashion house's finale' is the wordplay.
'fashion' becomes 'fad' (fad is a kind of fashion**).
'finale' suggests the final letters.
The last letter of 'house' is 'e'.
'fad'+'e'='FADE'
(Other definitions for fade that I've seen before include "Grow dim" , "Gradually grow faint" , "Discolour" , "Grow dimmer" , "Deaf (anag.)" .)