Key bacteria finally found on dead mushroom (8)
I believe the answer is:
escalate
'mushroom' is the definition.
(I've seen this before)
'key bacteria finally found on dead' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'finally' indicates one should take the final letters.
'found on' is a charade indicator (letters next to each other).
'dead' becomes 'late' (similar in meaning).
The final letter of 'bacteria' is 'a'.
'esc'+'a'+'late'='ESCALATE'
(Other definitions for escalate that I've seen before include "Increase in intensity or extent, like trouble brewing" , "Increase in extent or intensity (8)V" , "Cause to increase, say dispute (8)." , "Tease Cal badly and mount up" , "Magnify" .)