Symbol of life (4)
'symbol of life' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ankh that I've seen before include "Sign of life" , "Ancient Egyptian symbol of life - khan (anag)" , "Hank (anag) - cross symbolising eternal life" , "Waylay" , "Cross with a loop at the top, symbol of life in ancient Egypt" .)
(Other definitions for heartbeat that I've seen before include "Essential pulsation" , "Circulation noise" , "Pulse of cardiac origin" , "Animating force" .)