What I should have when small
I believe the answer is:
dot
'small' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for dot that I've seen before include "Full stop, period or tittle" , "scatter" , "Internet punctuation mark" , "Morse symbol; spot" , "Speck or period" .)