The term FENCE-SITTER'S DETERRENT can be solved by the crossword clue 'BARB' which is a four letter word. A BARB is a sharp point or hook on the end of an arrow, fishhook, or similar object, which is used to make it harder to remove from its target. In this context, a fence-sitter's deterrent would be something that makes it difficult to remain neutral or undecided, such as a strong argument or persuasive evidence.