Mistake by bully is a common bloomer (7)
I believe the answer is:
cowslip
'common bloomer' is the definition.
'cowslip' can be an answer for 'bloomer' (I've seen this before). I am not certain of the 'common' bit.
'mistake by bully' is the wordplay.
'mistake' becomes 'slip' (a slip is a blunder or mistake).
'by' means one lot of letters go next to another.
'bully' becomes 'cow' (I've seen this before).
'slip' after 'cow' is 'COWSLIP'.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for cowslip that I've seen before include "Yellow wild flower" , "Yellow field flower" , "Wild spring flower" , "Yellow flower for unsteady bovine?" , "Yellow spring flower" .)