Unambitious in home improvements, sadly (8)
I believe the answer is:
dismally
'sadly' is the definition.
(thesaurus)
'unambitious in home improvements' is the wordplay.
'unambitious' becomes 'small' (as in thinking small).
'in' indicates putting letters inside.
'home improvements' becomes 'diy' (DIY is do-it-yourself work done on the home).
'small' put into 'diy' is 'DISMALLY'.
(Other definitions for dismally that I've seen before include "lacking in cheer" , "Badly" , "In gloomy way" , "not very well" , "How one may fail" .)