Old ass, upset, leaves in stew? (7)
I believe the answer is:
oregano
'leaves in stew?' is the definition.
(type of herb)
'old ass upset' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ass' becomes 'onager' (type of wild ass).
'upset' is a reversal indicator.
'onager' back-to-front is 'regano'.
'o'+'regano'='OREGANO'
(Other definitions for oregano that I've seen before include "Wild marjoram" , "food flavouring" , "part of Mediterranean diet?" , "Herb - a no-goer (anag)" , "Aromatic culinary herb" .)