Bully knocked over beer, making a bloomer (7)
I believe the answer is:
cowslip
'a bloomer' is the definition.
(I've seen this before)
'bully knocked over beer' is the wordplay.
'bully' becomes 'cow' (I've seen this before).
'knocked over' says the letters should be written in reverse.
'beer' becomes 'pils'.
'pils' in reverse letter order is 'slip'.
'cow'+'slip'='COWSLIP'
'making' is the link.
(Other definitions for cowslip that I've seen before include "bloomers cover this" , "Flowering plant" , "Pretty little wild flower for unsteady bovine" , "Early spring flower" , "Wild spring flower" .)