Cash dispenser containing nothing or tiny amount (4)
I believe the answer is:
atom
'tiny amount' is the definition.
(I've seen this in another clue)
'cash dispenser containing nothing' is the wordplay.
'cash dispenser' becomes 'at m' (synonyms).
'containing' indicates putting letters inside.
'nothing' becomes 'O' (O resembles 0).
'atm' enclosing 'o' is 'ATOM'.
'or' is the link.
(Other definitions for atom that I've seen before include "Neutron and proton" , "Basic unit of a chemical element" , "The tiniest particle" , "A wee bit" , "Something very small perhaps" .)