Alfred picked up expression of sadness about King in sudden gathering (5,3)
I believe the answer is:
flash mob
'gathering' is the definition.
(I have seen 'Unexpected gathering' mean 'flash mob' so perhaps 'gathering' could also mean 'flash mob')
'alfred picked up expression of sadness about king in sudden' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is located in the answer.
'about' could be 'a' (abbreviation in Chambers) and 'a' is found in the answer.
'king' could be 'hm' (short for His Majesty) and 'hm' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for flash mob that I've seen before include "Sudden mass of people" , "Ad hoc group assembled to do something unusual in a public place" , "Unexpected gathering" , "streakers?" .)