Small quantity of margarine returned (4)
I believe the answer is:
gram
'small' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'quantity of margarine returned' is the wordplay.
'quantity' becomes 'G' (G is an example).
'of margarine' becomes 'mar' (I can't explain this - if you can you should give a lot more credence to this answer).
'returned' says the letters should be written backwards.
'mar' written backwards gives 'ram'.
'g'+'ram'='GRAM'
Can you help me to learn more?
(Other definitions for gram that I've seen before include "Chickpeas; unit of mass" , "Metric unit of weight" , "Small metric weight" , "pulses" , "Small unit of mass" .)