Failure to concentrate missing out at end (9)
I believe the answer is:
Here is my best explanation:
'end' is the definition.
(intention is a kind of end)
'failure to concentrate missing out at' is the wordplay.
'failure to concentrate' becomes 'inattention' (I've seen this before).
'missing out' is a deletion indicator.
'inattention' with 'at' taken away is 'INTENTION'.