Amongst microbes tested in the cream (4)
I believe the answer is:
best
'the cream' is the definition.
(thesaurus)
'amongst microbes tested in' is the wordplay.
'amongst' says the answer is hidden in the clue.
'BEST' is hidden within 'microbes tested in'.
(Other definitions for best that I've seen before include "Limit of one's ability" , "Superlatively good" , "Of the highest quality" , "Most advantageous" , "Overcome" .)