Grow, add to (8)
I believe the answer is:
increase
I believe this is a double definition.
'grow' is the first definition.
(I know that grow is a more specific form of the action increase)
'add to' is the second definition.
(I've seen this before)
(Other definitions for increase that I've seen before include "Wax" , "Become greater in size" , "Make larger in a sincere way" , "boost" , "What's more" .)