Origins of appallingly ripe odour making awful smell (5)
I believe the answer is:
aroma
'smell' is the definition.
(aroma is a kind of smell)
'origins of appallingly ripe odour making awful' is the wordplay.
'origins of' indicates taking the first letters.
The initial letters of 'appallingly ripe odour making awful' is 'AROMA'.
(Other definitions for aroma that I've seen before include "Odour, mostly pleasing" , "A pleasant smell as of food" , "Subtle quality" , "Distinctive odour" , "Pleasant odour" .)