Gullible person (3)
I believe the answer is:
sap
'gullible person' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sap that I've seen before include "Plant liquid or general vitality" , "Deplete, drain of resources" , "Drain - vital fluid" , "Plant juice for silly ass" , "The juice in trees" .)