When you do this, stop people drinking too much (8)

'people drinking too much' is the definition.
(tosspot is an informal term for a drinker)

'when you do this stop' is the wordplay.
I cannot really see how this works, but
an anagram of 'stop' is 'spot' which is present in the answer.
The remaining letters 'toss' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...