Sets a tail on and ensures it doesn't happen again (4,2,3,2)
I believe the answer is:
puts an end to
'doesn't happen again' is the definition.
I can't judge whether this defines the answer.
'sets a tail on and ensures it' is the wordplay.
I cannot really see how this works, but
'sets' could be 'puts' ('put' can be a synonym of 'set') and 'puts' is found within the answer.
'a' could be 'an' and 'an' is found in the answer.
'tail' could be 'end' (the tail of something can mean the end) and 'end' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?