Having broken a personal record, though on the decline (4,4,4)
I believe the answer is:
past ones best
'on the decline' is the definition.
I can't tell whether this defines the answer.
'having broken a personal record' is the wordplay.
I cannot really understand how this works, but
'a' could be 'one' and 'one' is present in the answer.
'record' could be 'best' (similar in meaning) and 'best' is located in the answer.
The remaining letters 'pasts' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'though' is the link.
Can you help me to learn more?
(Another definition for past ones best that I've seen is " No longer as strong as one was".)