Commit a faux pas and leave out a good guy (4,1,5)
I believe the answer is:
drop a brick
'commit a faux pas' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'leave out a good guy' is the wordplay.
'leave out' becomes 'drop-' (synonyms).
'good guy' becomes 'brick' (I've seen this before).
'drop'+'a'+'brick'='DROP A BRICK'
'and' is the link.
Can you help me to learn more?
(Other definitions for drop a brick that I've seen before include "Make a tactless remark" , "put foot in it?" .)