The term 'requiring great effort' can be summarized by the word 'STRENUOUS, ' which has nine letters. STRENUOUS is defined as involving or requiring a lot of effort, energy, and determination. It is often used to describe physical activities that require a lot of effort, such as running a marathon or lifting heavy weights. It can also be used to describe mental activities that require a lot of effort, such as studying for an exam or writing a research paper.