Untrustworthy person (5)
I believe the answer is:
knave
'untrustworthy person' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knave that I've seen before include "one found amongst thieves" , "Rat" , "Jack - scoundrel" , "Varlet; Jack" , "Unscrupulous man; jack" .)