Small amount will react badly (5)
I believe the answer is:
trace
'small amount' is the definition.
(I've seen this in another clue)
'react badly' is the wordplay.
'badly' indicates an anagram (letters in the wrong order).
'react' anagrammed gives 'TRACE'.
'will' acts as a link.
(Other definitions for trace that I've seen before include "Find - tiny portion" , "Detect - track" , "Barely detectable amount" , "Skimpy garment" , "Locate by investigation" .)