Drastic reduction in sales oddly overlooked (5)
I believe the answer is:
acute
'drastic' is the definition.
(thesaurus)
'reduction in sales oddly overlooked' is the wordplay.
'reduction' becomes 'cut' (cut is a kind of reduction).
'in' is an insertion indicator.
'oddly overlooked' means one should take alternating letters.
The alternate letters of 'sales' are 'ae'.
'cut' put into 'ae' is 'ACUTE'.
(Other definitions for acute that I've seen before include "Critical: shrewd" , "Sharp as of mind - or pain" , "Sharp - sort of French accent" , "Sharp; type of accent" , "not seen in cafe here!" .)