. . . which this tiny bit represents? (4)
I believe the answer is:
atom
'which this tiny bit represents?' is the definition.
'atom' can be an answer for 'tiny bit' (I've seen this in another clue). I'm not sure about the remainder of the definition.
'bit represents?' is the wordplay.
I cannot really understand how this works, but
'bit' could be 'o' (resembles 0, a binary digit or bit) and 'o' is located in the answer.
The remaining letters 'atm' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Scrap" , "Not much substance" , "Moat (anag.)" , "Minute fragment" , "A mighty physical particle or bomb" .)