Person apt to say the wrong thing (4)

I believe the answer is:

liar

I'm a little stuck... Click here to teach me more about this clue!

'person' is the definition.
(liar is a kind of person)

I don't understand how the remainder of the clue works.

(Other definitions for liar that I've seen before include "Teller of untruths" , "one telling a story" , "Untruthful person (Billy in Schlesinger film)" , "Tale-teller" , "Teller of fibs" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app