Terribly tiny self-assembly boxes (8)
I believe the answer is:
dismally
'terribly' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?
'tiny self-assembly boxes' is the wordplay.
'tiny' becomes 'small' (similar in meaning).
'self assembly' becomes 'diy' (I can't justify this - if you can you should believe this answer much more).
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'small' going into 'diy' is 'DISMALLY'.
Can you help me to learn more?
(Other definitions for dismally that I've seen before include "How one may fail" , "Badly" , "In gloomy way" , "lacking in cheer" , "not very well" .)