Walls of residence with effective buttress (9)
I believe the answer is:
reinforce
'buttress' is the definition.
(I've seen this in another clue)
'walls of residence with effective' is the wordplay.
'walls of' says to hollow out the word (remove centre letters) (only the edge letters remain).
'with' is a charade indicator (letters next to each other).
'effective' becomes 'inforce' (synonyms).
'residence' with its centre taken out is 're'.
're'+'inforce'='REINFORCE'
(Other definitions for reinforce that I've seen before include "Buttress" , "provide support" , "increase support" , "Make stronger" , "give added support" .)