Indicate by your expression when you enter (8)
I believe the answer is:
register
'indicate' is the definition.
(registering is a kind of indicating)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for register that I've seen before include "Index" , "Show" , "Official list or record" , "(Put one's name down on) list" , "To express" .)