I had let a lucky break start and end in vain chatter (4,4)
I believe the answer is:
idle talk
'chatter' is the definition.
(I know that idle talk is a type of chatter)
'i had let a lucky break start and end in vain' is the wordplay.
I cannot really see how this works, but
'i' could be 'a' and 'a' is present in the answer.
'had' could be 'd' (in contractions e.g. 'I'd', 'you'd' etc) and 'd' is found in the answer.
an anagram of 'let' is 'etl' which is located in the remaining letters.
The remaining letters 'ilk' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for idle talk that I've seen is " It's foolish to say".)