An ill-tasting drink makes them hiss! (5)
I believe the answer is:
meths
'an ill-tasting drink' is the definition.
I can't judge whether this definition defines the answer.
'makes them hiss' is the wordplay.
'makes' indicates anagramming the letters.
'hiss' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'them' is an anagram of 'meth'.
'meth'+'s'='METHS'
Can you help me to learn more?
(Other definitions for meths that I've seen before include "unsuitable alcohol" , "Spirit" , "Methylated spirits (colloq.)" , "Violet-coloured alcohol" , "Fuel" .)