Stan upset tiny creatures (4)
I believe the answer is:
ants
'tiny creatures' is the definition.
(I've seen this in another clue)
'stan upset' is the wordplay.
'upset' indicates anagramming the letters.
'stan' anagrammed gives 'ANTS'.
(Other definitions for ants that I've seen before include "Small social insects" , "Termites" , "Busy little insects" , "Inhabitants of a formicary" , "24 [INSECT] s" .)