An ill-tasting drink makes them hiss! (5)
I believe the answer is:
meths
'an ill-tasting drink' is the definition.
I can't tell whether this definition defines the answer.
'them hiss' is the wordplay.
I cannot really understand how this works, but
an anagram of 'them' is 'meth' which is within the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'makes' acts as a link.
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" .)