Lessening (9)
(Other definitions for reduction that I've seen before include "Drop in price" , "cut" , "cutback!" , "Lessening" , "Decrease, diminution" .)
'lessening' is the definition.
(I know that shrinkage is a type of lessening)
This is the entire clue.
(Other definitions for shrinkage that I've seen before include "effect of melting?" , "Contraction" , "Asking her to get loss by wastage" , "getting smaller" , "Process of shrivelling or contracting" .)
(Other definitions for let-up that I've seen before include "Pause in intensity" , "Respite, relief" , "Relieving pause" , "Pause, temporary respite" , "Abatement" .)
'lessening' is the definition.
(abating is a kind of lessening)
This is all the clue.
(Other definitions for abating that I've seen before include "Getting less" , "Becoming less intense" , "Dying away, slacking off" , "on the decline" , "Slacking off, dyeing away" .)