Microscopic organisms evidently originating in part of central Asia (8)
I believe the answer is:
bacteria
'microscopic organisms' is the definition.
(I've seen this before)
'evidently originating in part of central asia' is the wordplay.
'originating' indicates taking the first letters.
'in' is an insertion indicator.
'part of central asia' becomes 'bactria' (historical region).
The initial letter of 'evidently' is 'e'.
'e' inserted into 'bactria' is 'BACTERIA'.
(Other definitions for bacteria that I've seen before include "source of disease" , "Single-cell organisims" , "I eat crab (anag)" , "These single-cell organisms are in bait race" , "Caberet I perform for the tiny organisms" .)