IPA: /ˈdrʌmˌbiːtə/
KK: /ˈdrʌmˌbiːtər/
A person who strongly promotes or advocates for a particular cause or idea, often with great enthusiasm.
The drumbeater for the environmental campaign rallied the community to take action.
Drumbeater → It is formed from "drum" (from Old English *druma*, meaning a percussion instrument) and "beater" (from the verb "beat", meaning to strike). The word "drumbeater" refers to a person who strikes or plays a drum.
Think of someone who 'strikes' ('beat') a 'drum' — that's what a drumbeater does.
No commonly confused words.