Fatberg sat about causing alarm (11)
I believe the answer is:
flabbergast
'alarm' is the definition.
(flabbergasted means alarmed or shocked)
'fatberg sat about' is the wordplay.
'fatberg' becomes 'flabberg' (I can't justify this - if you can you should believe this answer much more).
'about' indicates anagramming the letters.
'sat' with letters rearranged gives 'ast'.
'flabberg'+'ast'='FLABBERGAST'
'causing' acts as a link.
(Other definitions for flabbergast that I've seen before include "floor" , "stunning" , "Surprise greatly" , "Strike dumb" , "Astonish" .)