Very small amount includes foreign money arbitrarily (2,6)
I believe the answer is:
at random
'arbitrarily' is the definition.
(I've seen this in another clue)
'very small amount includes foreign money' is the wordplay.
'very small amount' becomes 'atom' (I've seen this in another clue).
'includes' indicates putting letters inside.
'foreign money' becomes 'rand' (currency of South Africa).
'atom' placed around 'rand' is 'AT RANDOM'.
(Other definitions for at random that I've seen before include "without prior arrangement" , "In no prearranged order" , "Haphazardly" , "Haphazard" , "willy-nilly" .)