The term PROTON PART can be solved by the five-letter word QUARK. A QUARK is a type of elementary particle that is one of the fundamental constituents of matter. Quarks are found in hadrons, which are particles composed of quarks held together by the strong force. Protons are one type of hadron, and they are composed of two up quarks and one down QUARK. Therefore, the answer to the crossword clue PROTON PART is QUARK.