Make a mistake and get the message (6)
I believe the answer is:
errand
'the message' is the definition.
(I've seen this before)
'make a mistake and' is the wordplay.
'make a mistake' becomes 'err' (to err is to make an error).
'err'+'and'='ERRAND'
'get' is the link.
(Other definitions for errand that I've seen before include "Useful journey" , "short shopping trip" , "Task, message" , "Job usually involving a short trip" , "Short trip to perform required task" .)