Do fully (5)
I believe the answer is:
ditto
'do' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ditto that I've seen before include "As per previous" , "The same thing" , "Two dots say same again" , "Likewise" , "Mark used to indicate repetition" .)