Show improvement in regard to past performance (6)
I believe the answer is:
reform
'show improvement' is the definition.
(I've seen this before)
'regard to past performance' is the wordplay.
I cannot really see how this works, but
'to' could be 'for' and 'for' is located in the answer.
The remaining letters 'rem' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for reform that I've seen before include "Show improvement" , "Go straight" , "Mend one's ways" , "Change one's ways (for the better?)" , "Correct(ion of) abuses" .)