Organised activity for many an American fool covered in excess fat (5,3)
I believe the answer is:
flash mob
'organised activity for many an american' is the definition.
I can't judge whether this definition defines the answer.
'fool covered in excess fat' is the wordplay.
'fool' becomes 'shmo' (I've seen this in another clue).
'covered in' is an insertion indicator.
'excess fat' becomes 'flab' (I've seen this before).
'shmo' going into 'flab' is 'FLASH-MOB'.
Can you help me to learn more?
(Other definitions for flash mob that I've seen before include "Ad hoc group assembled to do something unusual in a public place" , "Sudden mass of people" , "streakers?" , "Unexpected gathering" .)