Diminish gradually (8)
'diminish gradually' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dwindle that I've seen before include "Become steadily less" , "Grow less - waste away" , "Flag" , "Diminish gradually" , "Lessen; fade out" .)
(Other definitions for decrease that I've seen before include "Iron out the wrinkles" , "Reduction" , "Become smaller or fewer" , "cut back" , "Diminish, lessen" .)