Final answer:
In Andrew Clements' novel 'Frindle', the 'secret agents' take an oath to use and promote the word 'frindle' instead of 'pen'.
Step-by-step explanation:
The oath in Frindle that the "secret agents" take is a pledge the students make in support of using the word 'friendly' instead of 'pen'. This occurs in the children's novel Frindle by Andrew Clements, where the protagonist, Nick Allen, creates a new word for the common pen as part of a playful act that becomes a serious movement. The students vow to only use the word 'frindle' instead of 'pen' and to encourage its usage amongst others, marking their solidarity and commitment to the cause.