Umpire, old, with room for improvement (6)
I believe the answer is:
reform
'improvement' is the definition.
(reform is a kind of improvement)
'umpire old with room' is the wordplay.
'umpire' becomes 'ref' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'with' says to put letters next to each other.
'room' becomes 'rm' (I've seen this in other clues).
'ref'+'o'+'rm'='REFORM'
'for' acts as a link.
(Other definitions for reform that I've seen before include "Mend one's ways" , "Removal of abuses" , "Show improvement" , "Correct(ion of) abuses" , "Go straight" .)