Brewers buy bags of it (3)
I believe the answer is:
tea
'brewers buy' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'of it' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'ea' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'bags' is the link.
Can you help me to learn more?
(Other definitions for tea that I've seen before include "Beverage from leaves" , "Early evening meal" , "Assam or Darjeeling, for example" , "Dried leaf infusion" , "Shrub; drink" .)