A plain repair I made a mess of (7)
I believe the answer is:
prairie
'a plain' is the definition.
(type of plain)
'repair i made a mess of' is the wordplay.
'made a mess of' indicates an anagram.
'repair'+'i'='repairi'
'repairi' anagrammed gives 'PRAIRIE'.
(Other definitions for prairie that I've seen before include "Large open area of grassland in the US" , "Large tract of grassland in N America" , "Large grassland region" , "American plain" , "Extensive US grassland" .)