Reduce one’s share (3)
I believe the answer is:
cut
This clue could be a double definition.
'reduce' is the first definition.
('cut' can be a synonym of 'reduce')
'one's share' is the second definition.
'cut' can be an answer for 'share' (cut is a kind of share). I'm not sure about the 'one's' bit.
Can you help me to learn more?
(Other definitions for cut that I've seen before include "Slice, chop" , "Divide; intersect" , "Percentage" , "Excise" , "Truncate; slice" .)