birthday
An 8 letter word that means the inside of anything might be "interior." This word has 8 letters and would probably fit your crossword puzzle clue.
"Endorsement" is an 8-letter word that means official approval or support.
accurate
famished
transfer
allocate
friction
Egoistic
Isolated
The 8-letter "f" word in science that means force is "friction."
initiate
Darkness.