One's upset by what smells (4)
I believe the answer is:
nose
'what smells' is the definition.
(I've seen this in another clue)
'one's upset' is the wordplay.
'upset' is an anagram indicator.
'ones' with letters rearranged gives 'NOSE'.
'by' is the link.
(Other definitions for nose that I've seen before include "Cyrano's large feature" , "Faculty for detecting" , "Asian wind instrument -- use Teflon (anag)" , "Small margin of victory" , "blow it!" .)