A crab's disturbed by a beetle! (6)
I believe the answer is:
scarab
'a beetle' is the definition.
(I know that scarab is a type of dung beetle)
'a crab's disturbed' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'a'+'crabs'='acrabs'
'acrabs' with letters rearranged gives 'SCARAB'.
'by' is the link.
(Other definitions for scarab that I've seen before include "Beetle, sacred to the ancient Egyptians" , "Sacred beetle of ancient Egypt" , "Egyptian stone" , "Sacred dung-beetle" , "divine creature" .)