Tiniest bits of crumbs adorning squalid tool shed (4)
I believe the answer is:
cast
'shed' is the definition.
(I know that shed can be written as cast)
'tiniest bits of crumbs adorning squalid tool' is the wordplay.
'tiniest bits of' suggests taking the first letters (I've seen 'tiny bit' mean this).
The initial letters of 'crumbs adorning squalid tool' is 'CAST'.
(Other definitions for cast that I've seen before include "Select actors for play or film" , "Squint to see" , "Team of actors in a play" , "'Send forth, a spell perhaps (4)'" , "Form < > company" .)