Bug etc turns up in contents of meal prior to X-ray (9)
I believe the answer is:
bacterium
'bug' is the definition.
(thesaurus)
'etc turns up in contents of meal prior to x-ray' is the wordplay.
'turns up' is a reversal indicator.
'in' means one lot of letters goes inside another.
'contents of meal prior to x-ray' becomes 'barium' (a 'barium meal' might be eaten before an X-ray).
'etc' backwards is 'cte'.
'cte' going inside 'barium' is 'BACTERIUM'.
(Other definitions for bacterium that I've seen before include "Micro-organism that can cause disease" , "germ?" , "health risk?" , "Fundamental" .)