Gullible person among heirs apparent (3)
I believe the answer is:
sap
'gullible person' is the definition.
(I've seen this in another clue)
'among heirs apparent' is the wordplay.
'among' says the answer is hidden in the clue.
'SAP' can be found hidden inside 'heirs apparent'.
(Other definitions for sap that I've seen before include "sucker" , "Insidiously undermine" , "One is easily fooled" , "Undermine; fool" , "Plant's fluid" .)