Bully, almost injure, American bum (6)
I believe the answer is:
harass
'bully' is the definition.
(I've seen this in another clue)
'almost injure american bum' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'injure' becomes 'harm' (harming is a kind of injuring).
'american bum' becomes 'ass' (US English term for bottom).
'harm' with its final letter removed is 'har'.
'har'+'ass'='HARASS'
(Other definitions for harass that I've seen before include "Vex - persecute" , "Vex - pester" , "Bother and annoy continuously" , "Badger, antagonise" , "press" .)