Become angry, often before flop (4)
I believe the answer is:
flip
'flop' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for flip that I've seen before include "Not showing respect; turn over" , "Toss - impertinent" , "Toss over; egg-nog" , "hit the roof" , "Turn (over) - impudent" .)