Early form of atrioventricular valve (5)
I believe the answer is:
larva
'early form' is the definition.
(I've seen this before)
'of atrioventricular valve' is the wordplay.
'of' indicates the answer is hidden within the clue.
'LARVA' can be found hidden inside 'atrioventricular valve'.
(Other definitions for larva that I've seen before include "Juvenile form of insect" , "Insect's immature form" , "Woodworm" , "Immature stage of an insect" , "Grub, caterpillar stage" .)