Great act of generosity gives away odd bits to sister (5)
I believe the answer is:
large
'great' is the definition.
(I know that great can be written as large)
I cannot understand how the rest of the clue works.
(Other definitions for large that I've seen before include "Regal (anag)" , "broad" , "Strapping" , "Vast" , "Bumper" .)