Infinitesimal (9)
(Other definitions for minuscule that I've seen before include "A lower-case letter" , "Not important" , "not capital" , "unimportant" , "Fine" .)
(Other definitions for teeny that I've seen before include "Informally very small" , "Minuscule" , "Very small (informal)" , "Microscopic" , "Tiny" .)
'infinitesimal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for atomic that I've seen before include "Relating to minute matter (6)" , "Referring to tiny particles" , "Relating to nuclear energy" , "of bombs" , "Extremely small" .)