Straining does it no harm (3)
I believe the answer is:
tea
'straining' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'it no harm' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'no harm' becomes 'ea' (I can't explain this - if you can you should give a lot more credence to this answer).
't'+'ea'='TEA'
'does' is the link.
Can you help me to learn more?
(Other definitions for tea that I've seen before include "Earl Grey?" , "Shrub; drink" , "Light afternoon repast" , "Sort of garden, chest or service" , "Infused beverage" .)