Thumb stuck under a basic building block? (4)
I believe the answer is:
atom
'basic building block?' is the definition.
'thumb stuck under a' is the wordplay.
'thumb' becomes 'tom' (fairy tale character Tom Thumb).
'stuck under' says to put letters next to each other.
'tom' put after 'a' is 'ATOM'.
(Other definitions for atom that I've seen before include "Smallest unit of matter" , "An explosive particle as in bomb" , "The tiniest particle" , "A mighty physical particle or bomb" , "Elementary particle" .)