Heartless darkness I tempt to return, finding blood filters (9)
I believe the answer is:
glomeruli
'blood filters' is the definition.
(plural of glomerulus, type of blood vessel)
'heartless darkness i tempt to return' is the wordplay.
'heartless' suggests removing the centre.
'darkness' becomes 'gloom' (gloom is a kind of darkness).
'tempt' becomes 'lure' (synonyms).
'to return' says the letters should be written in reverse.
'gloom' with its middle taken out is 'glom'.
'i'+'lure'='ilure'
'ilure' in reverse letter order is 'eruli'.
'glom'+'eruli'='GLOMERULI'
'finding' acts as a link.
(Other definitions for glomeruli that I've seen before include "Cluster of vessels" , "nerves?" .)