one advantage; a merit
Often used in the set phrase 一得一失 (いっとくいっしつ, 'one gain, one loss') or in contexts weighing pros and cons. As a standalone noun it is somewhat formal and less common in casual speech.
This plan has both an advantage and a disadvantage.
彼の提案にも一得はあると思う。
I think his proposal also has a merit.
gaining some and losing some; advantages and disadvantages
Compound of 一 (one) and 得 (gain, advantage). The exact historical derivation is uncertain, but it follows a classical Chinese pattern of enumerating gains and losses.