Crossword puzzle clue "invisible something a spy might write with"

Invisible ___ (something a spy might write with)

"To write is to write is to write . . ." writer

Spot to spy "Spy vs. Spy"

What a spy might write or crack

Something your poetry teacher might assign you to write about a particularly inspirational poetry teacher you've had *hint* *hint

He sang (but did not write) "I Write the Songs"

It may be used to write down a write-off

Something a spy might fly under