The insect that toppled a dynasty (4)
I believe the answer is:
gnat
'the insect' is the definition.
(gnat is a kind of insect)
'toppled a dynasty' is the wordplay.
'toppled' is a reversal indicator (in a down clue, letters go up).
'a dynasty' becomes 'tang' (Chinese dynasty).
'tang' backwards is 'GNAT'.
'that' acts as a link.
(Other definitions for gnat that I've seen before include "Something with bite" , "Stinging insect" , "Insect like small mosquito" , "insect's" , "a little bit of a nuisance" .)