Gullible one taken in by rogue loses thousand (5)
I believe the answer is:
naive
'gullible' is the definition.
(similar in meaning)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for naive that I've seen before include "Gullible, too trusting" , "Simple-minded" , "Unaffectedly trusting" , "Without worldly wisdom" , "Callow" .)