Mixture of bleach and lye gives you stomach pain (9)
I believe the answer is:
bellyache
'stomach pain' is the definition.
'mixture of bleach and lye' is the wordplay.
'mixture of' indicates an anagram (I've seen this in other clues).
'and' means one lot of letters go next to another (I've seen this in other clues).
'bleach' after 'lye' is 'lyebleach'.
'lyebleach' anagrammed gives 'BELLYACHE'.
'gives you' acts as a link.
(Other definitions for bellyache that I've seen before include "Gripe" , "Beef" , "complain" , "protest" , "Moan noisily" .)