Very surprised when hedonist collapses (10)
I believe the answer is:
astonished
'very surprised' is the definition.
(I've seen this before)
'when hedonist collapses' is the wordplay.
'when' becomes 'as'.
'collapses' is an anagram indicator.
'hedonist' with letters rearranged gives 'tonished'.
'as'+'tonished'='ASTONISHED'
(Other definitions for astonished that I've seen before include "Confounded" , "dumbfounded" , "Greatly surprised" , "Staggered" , "Amazed" .)