Improve a repair (5)
I believe the answer is:
amend
'improve' is the definition.
(I know that improve can be written as amend)
'a repair' is the wordplay.
'repair' becomes 'mend' (synonyms).
'a'+'mend'='AMEND'
(Other definitions for amend that I've seen before include "Correct (one's ways)" , "Change for the better" , "Make minor improvements to (a document)" , "Make corrections to (eg, document)" , "Make minor improvements to (something)" .)