Section of mounted slide bore a nasty bug? (8)
I believe the answer is:
anaerobe
'bug?' is the definition.
(thesaurus)
'section of mounted slide bore a nasty' is the wordplay.
'mounted' shows that the letters should be reversed in order (in a down clue, letters mounted up).
'section of' says the answer is hidden in the clue.
'eboreana' is hidden in the letters of 'slide bore a nasty'.
'eboreana' in reverse letter order is 'ANAEROBE'.
(Other definitions for anaerobe that I've seen before include "One can do without oxygen" , "possible bacterium" , "Tiny organism" , "specialised organism" , "Organism growing without free oxygen" .)