Be dishonest with money, that is (3)
I believe the answer is:
lie
'be dishonest' is the definition.
(I've seen this in another clue)
'with money that is' is the wordplay.
'with' is a charade indicator (letters next to each other).
'money' becomes 'l' (L can mean 'pounds').
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'l' next to 'ie' is 'LIE'.
(Other definitions for lie that I've seen before include "Falsity" , "Fabrication" , "Rest; falsehood" , "False statement" , "A white one is considered harmless" .)