Dropping introduction to brief is being dishonest (5)
I believe the answer is:
lying
'dishonest' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for lying that I've seen before include "Perjury" , "Deviating from truth" , "Telling untruths" , "Prone - mendacity" , "Extending" .)