One in quartet providing nothing in support of activist group (5)
I believe the answer is:
cello
'support of activist group' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'one in quartet providing nothing' is the wordplay.
'one in quartet providing' becomes 'cell' (I can't justify this - if you can you should believe this answer much more).
'nothing' becomes 'O' (O resembles 0).
'cell'+'o'='CELLO'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for cello that I've seen before include "Casals' instrument" , "big fiddle?" , "It's bigger than a viola, but smaller than a double bass" , "Jacqueline du Pre's instrument" , "Instrument in the violin family" .)