Blow, shattering blow, causes depression (9)
I believe the answer is:
punchbowl
'depression' is the definition.
(I have seen 'deep depression' mean 'punchbowl' so perhaps 'depression' could also mean 'punchbowl')
'blow shattering blow' is the wordplay.
'blow' becomes 'punch' (punch is a kind of blow).
'shattering' is an anagram indicator.
'blow' with letters rearranged gives 'bowl'.
'punch'+'bowl'='PUNCHBOWL'
'causes' acts as a link.
(Other definitions for punchbowl that I've seen before include "Drinking vessel" , "One can hold a lot of drink" , "One offers drink" , "where one mixes at a party" , "Source of drink" .)