Clue: Heartless darkness I tempt to return, finding blood filters (9)
Answer: GLOMERULI
Explanation: Definition: blood filters
A charade of GL[o]OM ('darkness') minus the middle letter ('heartless') plus ERULI, a reversal ('to return') of I LURE ('I tempt'); for a term new to me (and, I suspect, many others as well), the capillaries that filter the blood in the kidneys.