Dwindling of pile noted unhappily (9)
I believe the answer is:
depletion
'dwindling' is the definition.
'pile noted unhappily' is the wordplay.
'unhappily' is an anagram indicator.
'pile'+'noted'='pilenoted'
'pilenoted' is an anagram of 'DEPLETION'.
'of' acts as a link.
(Other definitions for depletion that I've seen before include "Reduction in number or quantity as through over-use" , "Reduction in total value (of a resource)" , "An exhausting, using up" , "consumption?" , "Emptying" .)