Acid spattered under wheels of an organ (7)
I believe the answer is:
cardiac
'of an organ' is the definition.
(relating to the heart, an organ)
'acid spattered under wheels' is the wordplay.
'spattered' indicates an anagram.
'under' is a charade indicator (letters next to each other).
'wheels' becomes 'car' (car wheel is a kind of wheel).
'acid' anagrammed gives 'diac'.
'diac' after 'car' is 'CARDIAC'.
(Other definitions for cardiac that I've seen before include "Physiologically, of the heart" , "Relating to the heart" , "Pertaining to the heart" , "Sort of surgery" , "Appertaining to the heart" .)