Strengthen or support (9)
I believe the answer is:
reinforce
This clue is a double definition.
'strengthen' is the first definition.
(reinforcing is a kind of strengthening)
'support' is the second definition.
(I've seen this before)
(Other definitions for reinforce that I've seen before include "give added support" , "Give added strength to" , "provide support" , "Strengthen or support with confrerie" , "Buttress" .)