In wretched fashion, baby consumed by putting up shelves etc (8)
I believe the answer is:
dismally
'in wretched fashion' is the definition.
'baby consumed by putting up shelves etc' is the wordplay.
'baby' becomes 'small'.
'consumed by' is an insertion indicator.
'putting up shelves etc' becomes 'diy' (abbreviation for do-it-yourself work).
'small' placed into 'diy' is 'DISMALLY'.
(Other definitions for dismally that I've seen before include "lacking in cheer" , "In gloomy way" , "Badly" , "not very well" , "How one may fail" .)