The term PRO OPPOSER can be broken down into two words, PRO and OPPOSER. PRO is an abbreviation for 'PROFESSIONAL' or 'in favor of, ' while OPPOSER is a noun meaning 'one who opposes.' The crossword solutions for this term are ANTI (4 letters) and CON (3 letters). ANTI is a prefix meaning 'AGAINST' or 'opposed to, ' while CON is a prefix meaning 'AGAINST' or 'opposing.' Therefore, the term PRO OPPOSER can be defined as someone who is professionally in favor of opposing something.