Make or become less (5)
'make or become less' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for decrease that I've seen before include "Contract" , "Become smaller or fewer" , "Weaken" , "Reduce in size" , "Grow smaller or less" .)
(Other definitions for abate that I've seen before include "Lessen - cease" , "Ease off" , "(Flood) recede" , "Go down like storm" , "Die down, slacken off" .)