With which to do a chemical test on a public toilet? Not quite (7)
I believe the answer is:
reagent
'with which to do a chemical test' is the definition.
(a reagent might be used in test)
'on a public toilet? not quite' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'public toilet?' becomes 'gents' (gentlemen's toilet).
'not quite' means to remove the last letter.
'gents' with its last letter taken away is 'gent'.
're'+'a'+'gent'='REAGENT'
(Other definitions for reagent that I've seen before include "Chemical with known properties" , "will cause some reaction" , "Get near (anag) - substance used in chemistry" , "Chemically active substance" , "Chemical substance used in tests" .)