Swell to head off violent larcenies (8)
I believe the answer is:
increase
'swell' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for increase that I've seen before include "Grow in size" , "Grow greater" , "Become greater in number" , "Add, make bigger" , "Make larger in a sincere way" .)