Build on (3)
'build on' is the definition.
(I've seen this in another clue)
This is the entire clue.
'build on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for add that I've seen before include "Reckon" , "Combine (numbers)" , "Reason for difficulty learning" , "Contribute" , "Go further" .)