The term 'hard to understand' can be described by several crossword solutions, such as 'OPAQUE' (6 letters), 'ABSTRUSE' (8 letters), 'INTRICATE' (9 letters), 'ARCANE' (6 letters), and 'clearasmud' (10 letters). 'OPAQUE' means not allowing light to pass through, and figuratively, something that is difficult to understand. 'ABSTRUSE' refers to something that is difficult to comprehend due to its complexity. 'INTRICATE' means something that is complicated and detailed. 'ARCANE' means something that is known or understood by only a few people. Lastly, 'clearasmud' is an informal term that means something that is difficult to understand.