Performance space met and can't improve (9)
I believe the answer is:
enactment
'performance' is the definition.
(I've seen this before)
'space met and can't improve' is the wordplay.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'and' means one lot of letters go next to another.
'improve' is an anagram indicator.
'met' put after 'cant' is 'cantmet'.
'cantmet' with letters rearranged gives 'actment'.
'en'+'actment'='ENACTMENT'
(Other definitions for enactment that I've seen before include "Command" , "validation" , "Legal process" , "Law that's been passed" , "A played-out" .)