Clue: Which might reveal one's inner thoughts of careless penmanship? (7,4)
Answer: INKBLOT TEST
Explanation: Definition: Which might reveal one's inner thoughts
This is what I call a sesquidef, or a definition-and-a-half: the definition refers to the Rorschach test, which purports to reveal things about people's personality through their reactions to a set of inkblot patterns