Make a mistake getting back into arrears (3)
I believe the answer is:
err
'make a mistake' is the definition.
(to err is to make an error)
'back into arrears' is the wordplay.
'back' is a reversal indicator.
'into' indicates a hidden word.
'rre' is hidden in the letters of 'arrears'.
'rre' backwards is 'ERR'.
'getting' acts as a link.
(Other definitions for err that I've seen before include "Miss the mark" , "Be incorrect" , "Be inaccurate" , "do something you shouldn't" , "Miscalculate" .)