Happy about the hissing when a drink's carbonated (5)
I believe the answer is:
gassy
'carbonated' is the definition.
(I've seen this before)
'happy about the hissing when' is the wordplay.
'happy' becomes 'gay' (similar in meaning).
'about' indicates putting letters inside.
'the hissing when' becomes 'ss' (I can't explain this - if you can you should give a lot more credence to this answer).
'gay' placed around 'ss' is 'GASSY'.
'a drink's' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gassy that I've seen before include "Fizzy" , "Verbose" , "Carbonated" .)