Fortify, buttress (10)
I believe the answer is:
strengthen
I believe this is a double definition.
'fortify' is the first definition.
(synonyms)
'buttress' is the second definition.
(thesaurus)
(Other definitions for strengthen that I've seen before include "Reinforce, substantiate" , "Bolster" , "Build up" , "Make more powerful" , "Make tough" .)