It appears as well in the original solution (4)
I believe the answer is:
also
'it appears as well' is the definition.
The definition suggests a singular noun but the answer is not.
'in the original solution' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALSO' can be found hidden inside 'original solution'.
Can you help me to learn more?
(Other definitions for also that I've seen before include "Likewise" , "To boot" , "on top" , "Plus" , "item's rarely used now for this" .)