One not to be trusted putting bar back (4)
I believe the answer is:
liar
'one not to be trusted' is the definition.
(I've seen this in another clue)
'bar back' is the wordplay.
'bar' becomes 'rail' (rail is a kind of bar).
'back' says the letters should be written backwards.
'rail' reversed gives 'LIAR'.
'putting' acts as a link.
(Other definitions for liar that I've seen before include "Not a truth-sayer!" , "Fibber" , "Teller of fibs" , "Teller of untruths" , "Tale-teller" .)