Effluvium uses needles in return (5)
I believe the answer is:
stink
'effluvium' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'uses needles in return' is the wordplay.
'uses needles' becomes 'knits' (I've seen this in another clue).
'in return' is a reversal indicator (return can mean to go backwards).
'knits' reversed gives 'STINK'.
Can you help me to learn more?
(Other definitions for stink that I've seen before include "Stench" , "Foul smell" , "Pong" , "Smell - great fuss caused" , "song and dance" .)