Gullible person gets resin (3)
I believe the answer is:
sap
I believe this clue is a double definition.
'gullible person' is the first definition.
(I've seen this in another clue)
'resin' is the second definition.
(I know that resin can be written as sap)
'gets' is the link.
(Other definitions for sap that I've seen before include "Moisture in tree or informal dunce" , "Tree's blood" , "Insidiously undermine" , "Vigour, energy" , "Drain - vital fluid" .)